SEWAGE  SLUDGE 


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SEWAGE  SLUDGE 


TREATMENT  AND  UTILIZATION  OF  SLUDGE 


BY 

ALEXANDER  ELSNER 


THE  DRYING  OF  SLUDGE 


FR.  SPILLNER 

TRANSLATED  BY 

KENNETH  AND  ROSE  S.  ALLEN 


OPERATION  OF  MECHANICAL  SEWAGE  PLANTS 

BY 

FR.  SPILLNER  AND  MR.  BLUNK 

TRANSLATED   BY 

EMIL  KUICHLING,  M.  AM.  SOC.  C.  E. 

CONSULTING  ENGINEER 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES 


KENNETH  ALLEN,  M.  AM.  SOC.  C.  E. 

ENGINEER  METROPOLITAN   SEWERAGE    COMMISSION   OF   NEW  YORK 


McGRAW-HILL    BOOK   COMPANY 

239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

1912 


: 

A, 

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COPYRIGHT,  1912 
BY 
BOOK  COMPANY 


Printed  and  Electrotyped 

by  The  Maple  Press 

York,  Pa. 


PREFACE 

With  the  rapidly  increasing  number  of  sewage  treatment 
plants  in  the  United  States  and  the  development  of  new  methods, 
those  interested  in  the  subject  will  appreciate  the  valuable 
contribution  to  our  literature  on  the  troublesome  subject  of 
sludge  contained  in  the  following  monographs  by  Dr.  Eisner, 
Dr.  Spillner  and  Mr.  Blunk.  The  painstaking  experiments  and 
extended  observations  of  these  gentlemen,  carried  on  under  most 
favorable  circumstances,  enable  them  to  speak  with  exceptional 
authority  on  this  subject. 

Dr.  Eisner's  paper — "Die  Behandlung  und  Verwertung  von 
Klarschamm"1 — contains  a  large  fund  of  data  gleaned  from 
experience  with,  and  observation  of,  the  more  important  German 
plants  arid  those  of  England.  The  broad  scope  and  thorough 
treatment  are  characteristic  of  the  German  investigator. 

Dr.  Spillner's  paper,  entitled  "Die  Trochnung  des  Klar- 
schammes," 2  is  particularly  valuable  on  account  of  the  details 
of  the  results  accomplished  up  to  the  end  of  1909  in  the  operation 
of  the  plants  of  the  Emschergenossenschaft,  which  are  now 
receiving  so  much  attention  in  this  country.  Dr.  Spillner,  as 
chemist,  gives  this  information  at  first  hand. 

The  third  paper  comprises  Part  III  of  a  series  written  yet  more 
recently  by  Dr.  Spillner  and  Mr.  Blunk  on  "Results  of  the  Opera- 
tion of  Some  of  the  Mechanical  Sewage  Clarification  Plants  of 
the  Emscher  Association/'3  This  has  been  translated  by  Mr. 
Emil  Kuichling,  M.  Am.  Soc.  C.  E.  The  title  of  this  paper  is, 
"Examination  of  the  Sludge,  the  Liquid  in  the  Septic  or  Lower 
Chamber  of  the  Deep  Emscher  Tanks,  and  the  Water  Drained 
from  the  Wet  Sludge  on  the  Drying  Beds."  As  this  of  more 
recent  date  than  the  former  article  by  Dr.  Spillner  the  authors 
have  had  the  advantage  of  further  experience  in  the  operation  of 
tanks  of  the  Emscher  or  Imhoff  type,  as  well  as  of  the  comments 
and  criticisms  concerning  their  design  or  efficiency  that  have 

1  Fortschritte  der  Ingenieururissenschaften,  Zweite  Gruppe,  24  Heft,  Leipzig,  1910. 

2  Mitteilungen  aus  der  Koniglichen  Prufungsanstalt  fur  Wasserversorgung  und  Abwasser- 
beseitigung,  14  Heft,  Berlin,  1911. 

3  Technisches  Gemeindeblatt,  Vol.  XIII,  pp.  313-377 


241314 


vi  PREFACE 

been  brought  out  in  the  intervening  time.  Moreover,  Mr. 
Blunk,  as  operating  engineer,  adds  to  the  discussion  informa- 
tion derived  from  the  engineer's  point  of  view  concerning  their 
operation. 

Although  up  to  the  present  time  sludge  treatment  has  been 
accorded  little  attention  in  America  as  compared  with  Germany 
or  England,  this  will  be  demanded  more  and  more  hereafter. 
Some  really  creditable  work  has  been  done  in  this  direction,  how- 
ever, and  it  has  therefore  been  thought  desirable  to  add  some 
notes  on  the  characteristics  of  American  sewages  and  on  the  more 
important  results  reached  here  in  the  treatment  and  utilization  of 
sludge. 

For  the  greater  convenience  of  American  engineers  the  meas- 
ures given,  unless  otherwise  stated,  are  those  customarily 
employed  in  the  United  States:  the  gallon  being  the  United 
States  gallon  of  231  cu.  in.;  the  ton,  that  of  2000  Ibs.,  etc.;  but 
for  the  convenience  of  others  the  metric  measure  given  by  the 
authors  of  the  first  three  parts  are  also  stated. 

Acknowledgment  is  here  made  of  the  courtesy  of  the  city 
officials  and  others  who  have  furnished  data  concerning  the  works 
under  their  charge  and,  in  particular,  of  the  valuable  assistance 
rendered  by  Mr.  Emil  Kuichling  in  the  translation  of  obscure 
passages  in  the  original  papers  by  Drs.  Ing.  Eisner  and  Spillner. 

K.  A. 

NEW  YORK,  November  26,  1911 


CONTENTS 

PART  I 

TREATMENT  AND  UTILIZATION  OF  SLUDGE.     BY  ALEXANDER  ELSNER. 

PAGK 

Treatment  and  utilization  of  sludge 3 

I.  Introduction 3 

II.  Sludge,  its  composition  and  amount 7 

Detritus  from  grit  chambers 8 

Detritus  from  screening  plants 9 

Sludge  from  plain  sedimentation 9 

Grease  contained  in  sludge 10 

Sludge  from  chemical  precipitation 10 

Sludge  from  lignite  process 11 

Sludge  from  septic  tanks 11 

Digestion  of  sludge 11 

Sludge  from  contact  beds      12 

Sludge  on  irrigation  fields 12 

Influence  of  the  manner  of  treatment 13 

Amount  of  sludge 15 

III.  The  removal  of  sludge  from  clarification  tanks 22 

Removal  of  detritus  from  grit  chambers 25 

Removal  of  sludge  from  tanks,  wells  and  towers      27 

a.  Removal  with  interruption  of  operation 27 

b.  Removal  of  sludge  during  operation 35 

1.  Construction      . 35 

2.  Mechanical  contrivances  for  removing  sludge 

during  operation 44 

c.  Contrivances  and  conduits  for  conveying  sludge      .    .  50 

IV.  Reduction  of  the  water  in  sludge 54 

a.  Drying  in  the  air 56 

b.  Drying  by  filter  presses 64 

c.  De- watering  sludge  by  centrifugal  machines 69 

d.  Other  methods  of  reducing  the  water  in  sludge 77 

V.  Utilization  of  sludge       .    .  ' 81 

'  a.  Utilization  of  the  fertilizing  properties  of  sludge      ....  83 

1 .  The  use  of  wet  sludge  as  a  fertilizer      85 

2.  Utilization  of  de- watered  sludge  as  fertilizer   ...  91 

3.  Production  of  fertilizer  which  can  be  strewn  over 

the  ground 93 

b.  Complete  utilization  of  calorific  value  by  burning   ....  95 

c.  Production  of  gas 101 

ix 


x  CONTENTS 

PAGE 

d.  Extraction  of  grease 106 

e.  Various  other  methods  of  disposal      109 

VI.   Considerations  regarding  the  treatment  and  utilization  of  sludge 

in  the  choice  of  a  method  of  clarification Ill 

Concluding  remarks 116 

PART  II 

THE  DRYING  OF  SLUDGE.  A  REPORT  FROM  THE  SEWERAGE  DIVISION 
OF  THE  EMSCHER  ASSOCIATION.  KGL.  BAURAT  MIDDELDORF, 
CHIEF  ENGINEER,  DR.  ING.  IMHOFF,  DIVISION  SUPERINTENDENT. 
BY  DR.  ING.  FR.  SPILLNER.  ESSEN-RUHR. 

Introduction 121 

I.  The  drying  of  sludge 123 

Necessity  of  drying 123 

Methods  of  drying  different  kinds  of  sludge 130 

II.  Drying  of  fresh  sludge .  131 

III.  Drying  of  septic  tank  sludge 140 

IV.  Drying  of  Emscher  tank  sludge 143 

Experiments  with  draining 146 

Comparative  experiments  with  fresh  and  decpmposed  sludge.  152 

The  reasons  for  facility  of  drainage 159 

Experience  with  drainage  methods  in  large  plants   .....  164 

PART  III 

RESULTS  OF  THE  OPERATION  OF  SOME  OF  THE  MECHANICAL  SEWAGE 
CLARIFICATION  PLANTS  OF  THE  EMSCHER  ASSOCIATION.  BY  DR. 
ING.  F.  SPILLNER  AND  MR.  BLUNK. 

Introductory  note   .    .    .    .    : 172 

Measurements  of  the  sludge 173 

Examination  of  the  liquid  sludge .178 

Examination  of  the  drained  sludge 184 

Examination   of  the  liquid  drawn  from  the   septic   chamber   of   an 

Emscher  tank  and  the  drainage  water  from  the  sludge  beds   .    .    .    .186 
Yearly  costs 189 

PART  IV 

SLUDGE  TREATMENT  IN  THE  UNITED  STATES.     BY  KENNETH  ALLEN, 

M.  AM.  Soc.  C.  E. 

I.  American  sewage 195 

II.  Detritus  from  grit  chambers      199 

III.  Screenings 202 

IV.  Sludge  from  plain  sedimentation 210 

V.  Septic  tank  sludge 217 

VI.  Sludge  from  Emscher  tanks 224 


CONTENTS  xi 

PAGE 

VII.  Sludge  from  chemical  precipitation 230 

VIII.  The  disposal  and  utilization  of  sludge .  236 

1.  Disposal  of  night  soil  on  farms .  236 

2.  Dumping  at  sea 

3.  Application  to  the  land .  239 

4.  Filter-pressing 

5.  Drying  with  centrifugal  machines .  244 

6.  Recovery  of  colorific  value .  246 

IX.  Summary  and  conclusions 252 


TREATMENT  AND  UTILIZATION 
OF  SLUDGE 


BY 

ALEXANDER  ELSNER 

TRANSLATED  BY 

KENNETH  AND  ROSE  S.  ALLEN 


PREFACE 


The  following  treatise  deals  with  the  problem  of  treating  and 
utilizing  sludge. 

It  seems  an  opportune  time  to  present  this,  for  although  there 
has  been  great  activity  during  the  past  few  years  in  solving  the 
question  of  sludge  disposal,  yet  a  general  discussion  of  the  require- 
ments necessary  for  its  treatment  and  utilization  and  their 
realization  by  existing  methods  has  not  yet  been  published. 

In  many  cases  the  question  of  treating  and  utilizing  the  sludge, 
even  in  thoroughly  worked  out  projects,  has  been  left  open. 

This  work  may  be  of  assistance  to  engineers  who  are  designing 
such  plants  in  judging  and  deciding  these  matters. 

It  is  based  on  personal  observation  in  journeys  of  inspection, 
in  which  a  large  number  of  important  plants  for  treating  sludge 
were  examined,  also  upon  a  study  of  the  most  recent  literature, 
a  list  of  which  is  appended,  and  on  information  which  has  been 
placed  at  my  disposal  by  a  great  number  of  city  officials,  for 
which  many  thanks  are  due. 

ALEXANDER  ELSNER. 
DRESDEN,  March,  1910. 


VI 1 


TREATMENT  AND  UTILIZATION 
OF  SLUDGE 


CHAPTER  I 
INTRODUCTION 

The  satisfaction  felt  in  the  more  perfect  methods  of  sewage 
clarification  and  their  adaptation  to  different  kinds  of  sewage  has 
been  diminished  to  an  increasing  extent  by  the  question  of  the 
disposition  of  the  sludge  which  accumulates  in  the  vicinity  of  the 
works. 

In  1857  the  highest  sanitary  authority  of  England  proposed 
that  a  part  of  the  filth  in  sewage  be  removed  before  discharge 
into  streams  in  order  to  prevent  their  further  pollution  and  the 
intolerably  unsanitary  conditions  resulting  therefrom;  and  it  was 
then  that  the  sludge  question  first  arose,  i.e.,  the  question  of  its 
removal  and  the  disposal  of  the  filth  separated  from  the  liquid. 

Formerly  sewage  had  been  disposed  of  in  the  easiest  and 
cheapest  way  by  discharging  it  into  a  stream,  or,  in  a  few  in- 
stances, distributing  it  over  the  land  for  financial  gain,  while 
now  the  sludge  was  accumulated  in  the  neighborhood  of  the 
plant  without  considering  that  the  gain  in  sanitary  conditions 
was  more  than  offset  by  the  putrefying  masses  of  sludge  in  the 
thickly  settled  manufacturing  towns,  thereby  impairing  the 
health  of  the  inhabitants. 

The  farmers  did  not  make  use  of  the  sludge  as  had  been 
expected.  This  was  partly  because  they  discovered  that  its 
value  had  been  overestimated,  and  partly  because  of  an  increase 
in  manufactures,  whereby  they  were  driven  more  and  more 
to  truck  farming  in  those  populous  districts,  requiring  a  more 
expensive  fertilizer,  which  they  were  then  able  to  pay  for. 

Two  ways  have  been  attempted  to  reduce  this  nuisance.  A 
method  was  sought  to  make  the  sludge,  which  contained  much 
lime  after  the  prevailing  chemical  treatment,  transportable  by 
draining  off  a  part  of  the  water  before  putrefaction  set  in,  in 

3 


4  SEWAGE  SLUDGE 

order  that  its  use  might  not  be  confined  to  the  limited  number  of 
farmers  in  the  neighborhood  of  the  works,  and  so  that,  in  this 
more  portable  condition,  it  might  have  an  increased  value 
commercially.  Clarification  processes  were  sought  which  would 
promise  a  smaller  output  of  sludge  while  otherwise  equally 
efficient. 

The  rapid  spread  of  septic  treatment  may  be  attributed  to  an 
exaggerated  idea  of  the  reduction  of  sludge  which  was  antici- 
pated. A  further  advantage  .was  the  comparative  infrequency 
of  the  objectionable  process  of  cleaning  required  by  this  method. 
The  introduction  of  biological  methods,  which  seemed  at  once  to 
solve  the  difficulty  by  means  of  the  sludge-consuming  activity 
of  micro-organisms,  was  favored  by  the  difficulty  in  caring  for 
the  annually  increasing  quantities  of  sludge  due  to  chemical 
precipitation. 

The  assumption  that  the  amount  of  sludge  would  be  reduced 
by  70  per  cent,  or  even  90  per  cent.,  as  had  at  first  been  expected, 
in  septic  tanks,  was  shown  to  be  erroneous,  nor  was  the  difficulty 
of  caring  for  the  sludge  removed  by  biological  treatment;  for 
even  contact  beds  become  clogged  more  or  less  quickly,  according 
to  the  fineness  of  the  material  and  the  frequency  of  filling,  and 
must  then  be  taken  apart  so  that  the  sludge  can  be  washed  away. 
With  sprinkling  filters,  especially  when  made  of  coarse  material, 
the  necessity  for  taking  them  apart  does  not  occur  so  frequently, 
but  flakes  of  deposited  matter  are  washed  out  of  the  beds,  which 
usually  necessitates  the  placing  of  a  sedimentation  basin  in  the 
line  of  the  effluent  conduit.  It  has  been  found  that  the  greatest 
practicable  preparatory  clarification  by  sedimentation  tanks 
may  increase  the  cleansing  power  of  bacteria  beds  by  1  1/2  or 
2  times,  while  at  the  same  time  postponing  a  premature  accumu- 
lation of  sludge. 

This  is  also  true  of  sprinkling  filters  and  intermittent  land 
filters.  Here  it  is  especially  the  grease,  animal  fibers,  hair  and 
cellulose  which  form  a  felt-like  surface  sometimes  2  in.  (5  cm.) 
thick,  injuring  the  plant  life,  lessening  the  filtering  capacity,  and 
hindering  the  aeration  of  the  soil.  Removing  this  cover  is 
expensive  and  much  of  the  fertile  soil  is  lost.  Furthermore, 
much  larger  volumes  of  sewage  can  be  delivered  to  the  land  after 
thorough  preliminary  treatment  (English  estimates  give  5  times 
as  much  with  chemical  treatment,  10  times  as  much  with  biolog- 
ical treatment),  a  most  important  fact  in  consideration  of  the 


TREATMENT  AND  UTILIZATION  OF  SLUDGE       5 

oasinii1  area  of  available  land  and  its  increasing  value  accom- 
panying the  growth  of  cities.  It  is  not  only  in  sedimentation, 
septic  treatment  and  chemical  precipitation,  as  well  as  in  screen- 
ing plants  and  grit  chambers  where  sludge  naturally  accumulates, 
but  also  in  sprinkling  niters,  land  filters  and  contact  beds  that  it 
becomes  a  troublesome  factor. 

Two  qualities  render  sludge  particularly  troublesome  to  both 
the  technical  employees  and  to  those  living  near  the  plant,  and 
also,  on  account  of  the  high  cost  of  removal,  to  the  town  author- 
ities. These  are  the  tendency  to  putrefaction,  particularly  in 
warm  weather,  and  the  contained  water,  which  increases  its 
volume  and  adds  to  the  cost  of  transportation. 

In  particular  its  tendency  to  putrefy  quickly  in  warm  weather 
with  a  strong,  disagreeable  odor,  which  becomes  a  nuisance  not 
only  to  the  operatives  at  the  works  themselves,  but  also  to  the 
residents  of  the  neighborhood,  made  a  change  ever  more  impera- 
tive. This  is  easily  understood  when  one  remembers  that  by  far 
the  greatest  part  of  a  city's  filth  is  stored  near  the  clarification 
plants.  What  large  quantities  are  involved  may  be  seen  from 
the  fact  that  in  the  16  years  from  1887  to  1903,  930,000  cu.  yds. 
(711,000  cbm.)  of  solids  were  removed  from  the  sewage  of 
Frankfort.  Here,  indeed,  as  in  most  places,  a  further  accumula- 
tion of  sludge  might  be  avoided  by  its  use  as  a  fertilizer;  but  the 
annoying  odors  already  mentioned  cannot  thus  be  avoided  since 
the  great  proportion  of  water  calls  first  for  its  drying  out  in  the 
air.  Commonly,  however,  the  demand  for  fertilizer  is  not  great 
since,  especially  as  in  the  vicinity  of  towns  lacking  a  sewerage 
system,  the  supply  of  night-soil,  with  its  higher  fertilizing  power, 
may  supply  the  farmers'  needs.  Many  examples  made  it  clear 
that  in  planning  clarification  plants  the  greatest  attention  should 
be  given  to  the  disposal  of  the  sludge.  This  was  the  case,  not 
only  in  England,  where  the  sludge  nuisance  appeared  more 
pressing  on  account  of  the  chemical  treatment,  which  was  pre- 
ferred for  its  greater  removal  of  sludge  and  for  the  enhanced 
value  of  the  sludge  itself  due  to  the  addition  of  lime,  but  also  in 
Germany,  where,  decades  later,  similar  conditions  were  repro- 
duced on  a  smaller  scale. 

But  even  where  it  is  easy  to  dispose  of  the  sludge,  whether 
dried  or  wet,  there  is  occasion  for  further  treatment.  For,  as 
this  by-product  is  of  small  value  and  of  considerable  mass,  there 
should  be  an  effort  to  avoid  its  transportation  and  treatment, 


6  SEWAGE  SLUDGE 

especially  by  manual  labor,  which  increases  the  cost  to  an  un- 
necessary extent.  What  an  enormous  expense  may  result  is  seen 
in  London,  where  about  $238,000  (1,000,000  M.)  is  annually 
spent  in  carrying  the  sludge  to  sea  in  tank  steamers. 

In  Leipzig,  too,  the  annual  expense  of  handling  is  about 
$7100  (30,000  M.),  mainly  for  carting  off  the  dried  sludge. 

Efforts  to  improve  this  condition  have  been  made  in  two 
directions.  One  was  to  remove  the  sludge  and  to  simplify  and 
cheapen  its  transportation  to  drying  beds  or  places  of  utilization, 
and  in  particular  to  avoid  the  unhygienic  manual  labor.  One 
way  to  effect  this  is  to  give  the  tanks,  wells  and  towers  for  sludge 
the  best  possible  form;  and,  further,  to  install  machinery  and 
apparatus  for  the  automatic  removal  of  the  sludge,  or  to  operate 
the  plant  so  as  to  produce  the  least  possible  amount  of  sludge 
with  equal  clarification. 

Other  experiments  and  attempts  have  been  made  to  remove 
the  water  from  the  sludge  more  quickly  and  with  less  objection 
than  by  drying  in  the  open  air,  or  at  least  to  improve  upon  this 
method,  water  being  the  greatest  drawback  to  rendering  sludge 
of  value.  Above  all,  it  is  desirable  to  retain  the  fertilizing  prop- 
erties of  the  sludge,  its  fats  and  calorific  value,  and  in  this  way 
to  reduce  the  cost  of  treatment,  efforts  which  are  important  even 
from  the  agricultural  standpoint.  It  is  estimated  that 
$143,000,000  (600,000,000  M.)  are  annually  lost  by  failure  to 
utilize  the  nitrogen  in  sewage,  but  one-tenth  of  which  is  used. 
Although  these  figures  are  theoretical  and  perhaps  exaggerated, 
they  should  cause  one  to  reflect. 

These  considerations  for  simplifying  and  improving  sfudge 
disposal  and  utilizing  it,  or  at  least  attempting  to  do  so,  are  of 
great  importance  to  an  engineer  who  is  planning  a  disposal 
plant.  Disregard  of  these  matters  has  often  resulted  in  costly 
alterations,  or  even  a  complete  change  of  plan. 

Any  standardizing  of  sewage  treatment  should  be  strictly 
avoided  and  each  plant  designed  with  reference  to  the  particular 
local  conditions. 


CHAPTER  II 

SLUDGE.  ITS  COMPOSITION  AND  AMOUNT 


By  sludge  is  here  meant  all  the  residue  which  remains  after 
treatment  of  city  sewage  by  grit  chambers,  bar  screens  and  mesh 
screens,  tanks,  wells,  and  towers,  by  plain  sedimentation  or 
chemical  precipitation,  septic  tanks,  contact  beds  or  irrigation 
fields. 

Its  composition  and  amount  depend  upon: 

1.  The  composition  and  volume  of  the  sewage. 

2.  The  manner  of  collection. 

3.  The  method  of  treatment. 

4.  The  operation  of  the  plant. 

1.  The  amount  and  composition  of  sludge,  which  consists 
mainly  of  the  undissolved  matter  contained  in  the  sewage,  vary 
quite  as  much  as  the  character  of  the  sewage  in  different  towns. 

Even  the  amount  differs  very  greatly.  To  mention  but  two 
examples,  Paris  sewage  contains  1515  parts  per  million  (mg.  per 
liter)  of  undissolved  material,  but  that  of  Hanover  270  parts  per 
million  (mg.  per  liter). 

Those  cities  whose  inhabitants  have  low  standards  of  living 
and  use  but  little  water  per  capita,  have  a  very  concentrated 
sewage,  and  so,  in  general,  a  large  volume  of  sludge. 

Aside  from  this,  the  amount  of  trade  wastes  determines  to  a 
large  extent  the  character  of  the  sewage.  This  depends,  not  only 
upon  the  volume  of  the  trade  wastes,  which  sometimes  surpasses 
that  from  domestic  sources,  but  also  upon. the  addition  of  certain 
substances,  particularly  free  acids  and  salts  of  iron,  which  can 
convert  undissolved  into  soluble  material,  and  thus  effect  the 
amount  and  character  of  the  sludge.  Certain  industries  add 
substances  which  increase  the  putrescibility  of  the  sludge  or 
retard  its  drying,  such  as  textile  mills  which  give  it  a  fibrous, 
felt-like  character.  Others  add  large  quantities  of  grease  which 
may  determine  the  method  of  removal  or  treatment  of  the  sludge. 
Other  substances,  particularly  from  metal  works,  act  on  the 
sewage  and  sludge  as  a  disinfectant. 

7 


8  SEWAGE  SLUDGE 

The  daily  change  in  the  character  of  the  sewage  is  of  import- 
ance in  plants  where  the  sludge  is  removed  during  continuous 
sedimentation,  particularly  where  it  is  carried  immediately  to 
the  filter  press  for  further  treatment.  Not  only  is  its  character 
changed  but  also  its  volume. 

2.  The  system  of  sewerage  is  of  importance  in  so  far  as  the 
amount  and  character  of  the  sludge  is  concerned,  as  considerably 
more  mineral  matter  reaches  the  sewers  in  a  combined  system. 
The  amount  of  this  material  again  effects  the  character  of  the 
sludge,  particularly  its  percentage  of  moisture.     Large  quantities 
of  filth  are  washed  in  from  the  streets  during  heavy  rains  as  also 
by  the  cleaning  of  asphalt  and  wooden  pavements. 

In  towns  where  the  streets  are  chiefly  macadam  and  where  no 
catch  basins  are  provided,  in  order  to  collect  as  much  of  the  filth 
as  possible  at  one  point  outside  the  city,  especial  care  should  be 
taken,  in  planning  the  dimensions  of  the  grit  chamber  and  the 
means  for  cleaning  it,  on  account  of  the  large  amount  of  mineral 
matter  brought  down. 

If  the  sewage  passes  through  pumping  stations  and  long  force 
mains  or  traverses  long  distances  before  it  reaches  the  treatment 
plant,  much  of  the  suspended  matter  will  be  broken  up,  thus 
reducing  the  amount  retained  by  the  tanks  and  screens,  a  con- 
sideration of  especial  importance  where  the  clarification  is 
effected  by  these  means  only. 

3.  The  greatest  variation  in  the  volume  and  character  of  the 
sludge  is  due  to  the  method  of  clarification:  that  is,  the  method 
and  arrangements  by  which  the  separable  matter  is  removed 
from  the  sewage. 

Not  only  is  the  amount  of  the  sludge,  but  also  its  condition 
and  composition,  dependent  on  the  efficiency  of  the  process  of 
clarification. 


DETRITUS  FROM  GRIT  CHAMBERS 

The  sediment  removed  by  grit  chambers  is  composed  princi- 
pally of  inorganic  matter.  Its  putrescibility,  the  most  offensive 
quality  of  sludge,  is  therefore  slight,  as  well  as  the  amount  of 
water  contained.  This  varies  from  35  to  60  per  cent.  It  depends, 
aside  from  the  manner  of  cleaning  the  grit  chamber  and  the  fre- 
quent stirring-up  resulting  therefrom,  upon  its  content  of  organic 
matter,  and  this,  again,  upon  the  velocity  of  flow  provided.  Two 


TREATMENT  AND  UTILIZATION  OF  SLUDGE       9 

inches  (5  cm.)  per  second  should  be  the  least.  On  the  one  hand 
the  attempt  should  be  made  to  keep  the  deposit  as  free  as  possible 
from  putrescible  organic  matter  in  order  to  permit  of  its  con- 
venient disposal,  its  handling  and  transportation;  on  the  other, 
it  should  be  remembered  that  the  mineral  particles  of  the  de- 
tritus carried  to  other  parts  of  the  'plant  form  there  an  undesir- 
able ballast  which  makes  the  care  and  utilization  of  the  accumu- 
lated sludge  difficult.  For  this  reason  the  installation  of  a  special 
grit  chamber  is  seldom  omitted. 

DETRITUS  FROM  SCREENING  PLANTS 

The  screenings  from  screening  plants  differ  greatly  both  in 
amount  and  character.  The  meshes  of  the  screen  or  the  spacing 
of  the  bars  may  run  from  1.0  in.  (25  mm.)  to  0.04  in.  (1  mm.) 
according  to  whether  it  is  desired  to  keep  coarse  matter  from 
the  sewage  or  sludge  pump,  or  to  secure  the  greatest  possible 
clarification  of  the  sewage. 

The  amount  changes,  also  in  the  course  of  the  day.  As  most 
of  the  coarser  suspended  matter  consists  of  wastes  from  habita- 
tions the  amount  reaches  its  maximum  at  noon  or  soon  after, 
and  almost  disappears  by  nightfall.  (  This  fact  is  of  importance  in 
the  case  of  .bar  screens.  The  material  is  almost  wholly  organic 
and  consists  of  scraps  of  meat,  vegetables  or  fruit,  cloth,  hair, 
corks,  wood  and  lumps  of  fecal  matter. 

Its  composition  varies  so  widely  that  it  is  impossible  to  give 
an  average  value.  The  amount  of  water  contained  is  small, 
amounting  to  but  70  or  80  per  cent.  On  account  of  its  organic 
origin  it  is  highly  putrescible. 

SLUDGE  FROM  PLAIN  SEDIMENTATION 

Sludge  formed  by  the  process  of  sedimentation  in  tanks,  well 
and  towers  consists  of  a  semi-liquid,  black  mass  which  soon  be- 
comes offensive  with  much  gas  and  foul  odors.  Its  decomposi- 
tion is  accelerated  by  warm  weather. 

The  amount  of  water  in  the  sludge  in  tanks  is  usually  90  per 
cent.,  in  wells  and  towers  95  per  cent,  and  more,  and  is  regulated 
by  the  manner  of  treatment. 

In  conjunction  with  the  sludge  from  chemical  precipitation 
and  septic  tanks,  it  produces  the  largest  volume  to  be  treated 
on  account  of  the  proportion  of  water  contained.  Therefore, 


10  SEWAGE  SLUDGE 

most  of  the  efforts  and  treatment  for  the  utilization  of  sludge 
aim  at  its  removal. 

GREASE  CONTAINED  IN  SLUDGE 

Here  we  see  the  significance  of  the  grease  contained  in  sewage 
and  also  in  the  sludge  as,  on  the  one  hand,  this  may  interfere 
with  the  use  of  the  sludge  in  agriculture  and,  on  the  other,  has 
led  to  attempts  for  the  recovery  and  utilization  of  the  grease. 

According  to  the  experiments  of  Schreiber,  the  Berlin  sewage 
contains  22  Ibs.  of  grease  per  1000  persons  (20  g.  per  capita) 
per  day,  i.e.,  16.1  Ibs.  (7.3  kg.)  per  capita  per  annum.  This 
corresponds  to  a  quantity  of  grease  in  the  sewage  of  from  0.01 
to  0.026  per  cent.,  most  of  which  reappears  in  the  sludge. 

Also,  in  the  settled  sludge  of  other  cities,  as  Frankfort-on-the- 
Main,  Mannheim,  Elberfeld  and  Cassel,  the  amount  of  grease  is 
found  to  be  from  15  to  20  per  cent,  of  the  dried  material. 

The  Kremer  apparatus  shows  much  larger  amounts  of  grease 
in  the  scum.  In  the  Osdorf  experimental  plant,  with  an  amount 
of  water  in  the  sludge  from  81  to  86  per  cent.,  they  found  from 
9  to  6  per  cent,  of  grease,  or,  referred  to  the  dried  material,  as 
much  as  49  per  cent.  In  Charlottenburg,  with  the  same  appara- 
tus, 12.8  Ibs.  of  grease  per  cubic  yard  (7583  g.  per  cbm.)  was 
recovered.  This  would  give  500  Ibs.  (227  kg.)  of  grease  from 
7,925,000  gallons  (30,000  cbm.)  of  sewage  per  day  for  the  whole 
city. 

SLUDGE  FROM  CHEMICAL  PRECIPITATION 

The  sludge  from  chemical  precipitation  is  similar  in  character 
to  that  from  plain  sedimentation  but  the  quantity  is  much 
larger,  which  fact,  taken  in  connection  with  its  decreased  value 
as  a  fertilizer  on  account  of  chemicals  used  in  the  process,  has 
been  to  a  great  extent  the  reason  for  abandoning  this  method  of 
treatment. 

The  large  volume  of  sludge  is  explained  by  the  more  complete 
separation  of  the  undissolved  material  by  chemical  treatment, 
and  also  by  the  addition  of  the  precipitant. 

It  should  be  noted  that  2086  Ibs.  of  lime  per  million  gallons 
(250  g.  per  cbm.)  of  sewage,  which  is  often  used  in  England, 
produces  not  only  0.55  Ibs.  (250  g.)  of  sludge  but  5.52  Ibs. 
(2500  g.)  as  ,the  lime  settles  as  sludge  containing  about  90  per- 
cent, water. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     11 

SLUDGE  FROM  LIGNITE  PROCESS 

The  sludge  obtained  by  this  process  is  very  full  of  water  (95 
per  cent,  or  more)  but  can  generally  be  easily  pressed  or  dried  in 
the  air,  as  is  done  at  Copenick,  without  putrefaction  or  the 
emission  of  unpleasant  odors.  The  pressed  sludge  has  but  a 
faint  musty  smell  and  is  non-putrescible. 

SLUDGE  FROM  SEPTIC  TANKS 

The  same  thing  is  true  of  the  sludge  from  the  septic  tank,  as 
this  has  gone  through  the  process  of  decomposition  in  the  tank. 

This,  too,  has  only  a  slight  musty  odor. 

It  has  a  granular  earthy  structure  from  decomposition,  in 
contrast  with  precipitated  sludge  which,  after  the  water  is  drawn 
off,  has  a  fibrous,  felt-like  appearance. 

The  earthy  character  of  sludge  from  the  septic  tank  process 
aids  in  the  removal  of  the  water,  enabling  it  to  dry  more  rapidly. 
It  is  also  more  fluid,  in  comparison,  in  spite  of  the  small  amount 
of  contained  water — about  80  per  cent. 

All  the  data  thus  far  given  concerning  the  amount  of  water 
contained  in  sludge  are  averages  taken  from  a  large  number  of 
plants  and  are  subject  to  certain  variations  depending  upon  the 
design  of  the  plant  and  particularly  upon  the  thoroughness  of  the 
mode  of  operation  and  the  method  of  removing  the  sludge. 

'  DIGESTION  OF  SLUDGE 

In  the  septic  process  one  phenomenon  has  been  much  discussed, 
namely,  the  digestion  of  the  sludge.  That  is  to  say,  a  diminution 
of  the  quantity  of  the  sludge  in  such  a  manner  that  the  dried 
solids  contained  both  in  it  and  in  the  effluent  of  the  septic  tank 
are  less  than  the  amount  received.  A  reduction  of  80  per  cent,  or 
more  was  anticipated  by  the  introduction  of  the  septic  treatment 
and  thereby  relief  from  the  troubles  associated  with  sludge.  The 
amount  of  suspended  matter  contained  in  the  effluent  must  be 
considered  in  determining  the  percentage  of  reduction,  and  this 
is  rather  large.  In  different  English  cities,  for  example,  it  varies 
from  about  35  parts  per  million  (mg.  per  1.)  at  Salford  to  244 
parts  per  million  (mg.  per  1.)  at  Birmingham.  Moreover,  the 
sludge  becomes  more  dense  in  time  and  in  this  way  loses  a  part  of 
its  water.  In  fresh  sludge  this  is  about  90  per  cent,  and  in  dried 
sludge  80  per  cent.,  as  has  been  noted;  therefore  the  former  has 
twice  the  volume  of  the  latter. 


12  SEWAGE  SLUDGE 

Moreover,  a  part  of  the  solids  is  transformed  into  gas  and 
another  part  liquefies.  This  alone  is  to  be  considered  in  the 
reduction  of  sludge. 

The  amount  of  this  in  different  English  cities  is  as  follows: 
Birmingham  10  per  cent.,  Manchester  25  per  cent.,  Leeds  30  per 
cent.,  Sheffield  33  per  cent. 

In  Unna,  also,  where  it  was  removed  only  once  a  year  it  was 
found  to  be  but  about  60  per  cent,  of  the  aggregate  amount  when 
removed  weekly. 

The  reason  for  the  difference  between  these  several  figures  lies 
partly  in  the  difference  in  the  composition  of  the  sewage,  es- 
pecially whether  it  is  putrescible  domestic  sewage  or  whether  it 
contains  much  impalpable  mineral  or  other  material  not  easily 
broken  up,  and  partly  in  the  different  amounts  of  the  solids  con- 
tained in  the  effluent  of  the  septic  tank. 

A  certain  amount  of  sludge  reduction  may  always  be  expected 
after  prolonged  storage. 

SLUDGE  FROM  CONTACT  BEDS 

Sludge  is  also  found  in  contact  beds  and  its  removal  at  stated 
intervals  is  necessary,  depending  on  the  extent  of  its  preparatory 
treatment,  the  construction  of  the  beds  and  their  operation. 
The  sludge  that  is  washed  out  is  of  an  earthy  consistency,  con- 
tains 60  to  75  per  cent,  water  and  is  readily  dried.  As  it  is  bio- 
logically digested  it  does  not  become  foul  later  except  in  those 
cases  where  the  beds  are  overloaded.  It  strongly  resembles 
septic  sludge.  In  sprinkling  filters  it  contains,  under  certain 
conditions,  a  larger  amount  of  organic  matter,  such  as  the  larvae 
of  flies  and  mosquitoes,  while  in  contact  beds  it  contains  earth- 
worms and  other  worms. 

SLUDGE  ON  IRRIGATION  FIELDS 

Sludge  sometimes  appears  on  irrigation  fields  in  the  form  of  a 
layer  of  slime  which  covers  the  soil.  It  consists  of  cellulose  and 
grease,  prevents  the  admission  of  air  and  sewage  and  must  be 
removed  with  a  spade.  This  slime  can  be  avoided  by  installing 
sedimentation  tanks. 

The  differences  in  sludge  obtained  by  the  various  kinds  of 
treatments  mentioned  correspond  also  to  its  chemical  composi- 
tion, so  that  there  is  little  practical  value  in  giving  definite  limits 
to  the  amount  of  each  ingredient  of  which  it  is  composed. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     13 

In  order  to  present  a  general  idea  of  the  matter  some  analyses 
showing  results  obtained  with  different  methods  of  treatment 
are  here  given.  Later  some  materials  will  be  considered  more 
fully  which  give  to  sludge  a  certain  fertilizing  value. 

ANALYSES  OF  SLUDGE  IN  PER  CENT. 


Sludge  from 

Sludge  precipitated  by 

plain  sedimen- 
tation 

Sludge 

Septic 

from 

Frankfort-on- 
Main 

Lime 

Sulphate  of 
iron  and  lime 

Sludge. 
Stuttgart 

contact 
beds. 

Tempel- 

hof 

Wet 

Dry 

Frankfort-on-Main 

Water  

91.07 

90.85 

80.96 

77.3 

74.2 

Organic  matter  

5.08 

57.00 

4.15 

13.31 

7.35             16.5 

Nitrogen  contained  . 

0.23 

2.85 

0.31 

0.10 

0.4                 0.6 

Inorganic  matter  

3.85 

43.00 

5.00 

5.73 

15.35               9.3 

Phosphoric  acid  con-:     0.23 

2.85 

0.07 

0.02 

0.4                 0.5 

tained. 

Potash  :     0.05 

0.56        0.02 

0.007                 0.1 

Oxide  of  iron  



2.0 

INFLUENCE  OF  THE  MANNER  OF  TREATMENT 

4.  The  management  of  the  plant  has  a  marked  effect  on  the 
quality  of  the  sludge,  especially  the  amount  of  water  contained, 
and  therefore  upon  the  total  volume. 

As  already  shown,  this  is  the  case  to  a  certain  extent  with  grit 
chambers.  A  frequent  cleaning  out  of  the  sludge,  possibly  with 
dredges,  causes  a  stirring  up  of  the  deposit,  thus  mixing  it  with 
the  sewage,  while  in  plants  where  the  cleaning  is  done  at  longer 
intervals  after  cutting  out  the  grit  chamber,  which  is  ordinarily 
divided  into  compartments  built  side  by  side,  the  grit  deposits 
more  firmly  and  contains  less  water. 

This  difference  is  not  very  important  in  consideration  of  the 
comparatively  small  quantity  of  deposit  in  the  grit  chamber,  its 
easy  handling  and  its  inoffensive  character. 

In  settled  sludge,  on  the  other  hand,  the  quantity  of  water  is 
of  great  importance,  as  its  large  amount  may  readily  result  in  a 
nuisance  on  account  of  its  unforeseen  increase  as  well  as  from  the 
greater  cost  of  the  transport  and  disposal  of  the  increased  volume. 

In  considering  the  influence  of  the  contained  water  on  the 
amount  of  sludge  it  should  be  noted  that  1.3  cu.  yds.  (1  cbm.)  of 
sludge  having  80  per  cent,  moisture  contains  7  cu.  ft.  (200  1.)  of 


14  SEWAGE  SLUDGE 

dried  solids,  while  1.3  cu.  yds.  (1  cbm.)  of  sludge  having  95  per 
cent,  moisture  contains  1.75  cu.  ft.  (50  1.)  of  dried  solids;  there- 
fore the  amount  of  the  contained  solids  is  as  4  :1 .  A  given  volume 
of  sewage  from  which  the  suspended  matter  is  removed  and  dried 
produces  in  the  first  case  4  times  that  in  the  last.  The  amount 
of  this  material  alone  corresponds  to  the  degree  of  clarification, 
but  not  to  the  total  volume  of  the  sludge. 

Sludge  contains  the  most  moisture,  95  per  cent,  and  over,  in 
plants  where  it  is  removed  continuously,  as  in  this  case  it  is 
always  kept  in  motion  and  cannot  settle. 

In  clean  sedimentation  tanks  also,  i.e.,  where  great  care  is 
taken  to  prevent  the  settled  sludge  from  putrefying,  the  water 
content  is  large,  especially  in  summer,  as  the  sludge  must  be 
removed  frequently. 

In  order  to  reduce  the  volume  of  sludge  it  may  be  allowed  to 
remain  longer  and  partially  digest  when  the  effluent  from  the 
sedimentation  tank  is  given  subsequent  treatment,  possibly  on 
contact  beds  of  irrigation  fields.  Sludge  is  then  produced  with 
85  to  90  per  cent,  less  moisture. 

It  is  assumed  that  the  sewage  does  not  contain  substances  from 
industrial  plants  which  inhibit  putrefaction,  so  that  it  is  at  most  a 
question  of  reducing  the  volume  by  consolidation  from  prolonged 
settling. 

That  the  consolidation  of  sludge  has  an  effect  on  its  water  con- 
tent is  seen  in  the  Emscher  tank,  where  the  sludge  at  a  depth  of 
36  ft.  (11  m.)  contains  70  per  cent,  moisture  and  at  29.5  ft.  (9m.) 
75  to  80  per  cent.  This  occurs  less  in  ordinary  tanks,  with  their 
less  depth  of  sewage  and  sludge,  than  in  wells  and  towers. 

The  velocity  of  the  sewage  also  affects  the  amount  of  water  in 
the  sludge.  This  has  been  demonstrated  by  the  experiments  of 
Steurnagel  at  Cologne.  It  was  found  that  the  slower  the  velocity 
in  the  tank  the  greater  the  volume  of  sludge. 

The  experiments  showed  that  in  264,170  gallons  (1000  cbm.)  of 
sewage  with  a  velocity  of  0.156  in.  (4  mm.)  per  second  the  result 
was  5.28  cu.  yds.  (4.04  cbm.)  of  sludge,  with  95.57  per  cent,  of 
moisture  and  4.43  per  cent,  of  dry  material;  with  .78  in.  (20  mm.) 
per  second  velocity,  3.23  cu.  yds.  (2.47  cbm.)  of  sludge,  with 
92.87  per  cent,  of  moisture  and  7.13  per  cent,  of  dry  material; 
with  1.56  in.  (40mm.)  per  second  velocity,  2.41  cu.  yds.  (1.84  cbm.) 
of  sludge,  with  91.34  per  cent,  of  moisture  and  8.66  per  cent,  of 
dry  material.  An  examination  of  these  figures  shows  that  the 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     15 

volume  of  sludge  is  greatly  increased  by  diminishing  the  velocity, 
but  that  the  amount  of  the  dried  material  is  less,  so  that  after  all 
a  much  larger  volume  is  produced  [twice  as  much  by  .16  in. 
(4  mm.)  per  second  velocity  as  with  1.56  in.  (40  mm.)]  without 
improving  the  clarification;  for  the  amount  of  the  dried  material 
removed  is  as  17.9:15.9.  These  experiments,  however,  only 
lasted  one  day.  In  practice  one  can  count  on  a  greater  con- 
solidation of  water  and  sludge  with  a  velocity  of  0.16  in.  (4  mm.) 
per  second. 

The  explanation  of  the  result  of  this  experiment  is  that  with 
the  greater  velocity  the  finer  particles  of  sediment,  which  are 
contained  in  a  large  quantity  of  water  but  which  altogether  com- 
prise but  a  small  amount  of  dried  solids,  are  carried  into  the 
receiving  stream. 

If  sedimentation  tanks  are  used  intermittently,  i.e.,  if  the 
sewage  is  retained  in  them  for  some  time  after  filling,  possibly 
from  2  to  6  hours,  and  is  then  drawn  off,  clarified,  the  amount 
of  settled  sludge  is  less  than  with  a  continuous  flow.  Sludge  is 
always  stirred  up  in  refilling  which  does  not  settle  again  but 
passes  out  in  the  effluent.  It  does  not  pay  to  clean  out  the 
comparatively  slight  deposit  from  every  filling,  in  which  way 
this  objection  could  be  met. 

In  London  at  Barking  the  proportion  of  sludge  derived  from 
intermittent  treatment  (with  2  hours'  resting)  to  that  from  con- 
tinuous flow  is  as  1:3,  but  this  is  in  part  due  to  the  short  time 
allowed  for  resting.  The  difference  in  the  volume  and  consistency 
of  the  sludge  is  marked  between  the  operation  of  several  tanks 
in  parallel  and  in  series. 

In  the  latter  case  a  large  amount  of  thick,  viscous  sludge  is 
deposited  in  the  first  tank  and  a  fine,  greasy  mass  of  less  volume 
in  the  last.  As  cleaning  out  is  therefore  required  less  frequently 
in  the  latter,  this  system  and  also  the  sludge  more  nearly  resem- 
ble those  of  septic  treatment  plants. 

In  planning  for  the  removal  and  utilization  of  sludge  and  the 
sums  to  be  expended,  the  method  of  clarification  should  be  con- 
sidered from  the  beginning. 

AMOUNT  OF  SLUDGE 

It  is  also  desirable  to  possess  more  information  concerning  the 
amounts  of  sludge  deposited  by  the  different  methods  of  clarifi- 
cation. 


16  SEWAGE  SLUDGE 

It  is  impossible  to  give  final  figures  or  formulae  by  which  the 
quantity  of  sludge  to  be  expected  may  be  exactly  estimated, 
for  this  must  vary  within  very  wide  limits  for  the  reasons  already 
stated.  Exact  values  can  only  be  ascertained  by  experiments 
with  the  sewage  in  question,  which  should  always  be  made  in  the 
case  of  large  plants  and  especially  where  the  sewage  shows  any 
peculiarity. 

However,  we  can  usually  judge  of  the  quantity  of  sludge  to  be 
expected  from  the  amount  of  suspended  matter  in  the  sewage. 

In  this  way  Busing,  starting  with  an  amount  of  suspended 
matter  of  about  700  parts  per  million  (g.  per  cbm.)  and  a  ratio 
of  mineral  to  organic  matter  of  2  :  3,  knowing  the  volume  of  the 
sewage  and  considering  the  amount  of  matter  retained  by  catch 
basins,  etc.,  estimates  an  amount  of  suspended  matter  equal  to 
1/1000  the  volume  of  the  sewage.  The  limiting  values  he 
states  as  1/750,  from  a  rather  concentrated  sewage,  to  1/3000, 
where  water  is  liberally  used.  This  corresponds  to  0.076 
to  0.307  cu.  in.  of  dried  material  per  gallon  (0.33  to  1.33  1.  per 
cbm.)  or  1.63  to  6.58  cu.  yds.  sludge  90  per  cent,  water  per 
million  gallons  (3.3  to  13.3  1.  per  cbm.)  and  0.43  to  1.74  cu.  yds. 
per  1000  persons  (0.33  to  1.33  1.  per  cap.)  daily  with  a  water  con- 
sumption of  26.4  gallons  (100  1.)  per  day.  With  an  assumed 
average  of  1  to  1500  for  normal  sewage  the  last  two  values  would 
be  33.2  cu.  yds.  of  sludge  per  million  gallons  (6.71  1.  per  cbm.), 
or  0.88  cu.  yds.  per  1000  persons  (0.67  1.  per  capita)  per  day. 

These  values  agree  well  as  to  the  contained  moisture  with  the 
figures  of  Imhoff  (Proceedings  of  the  Royal  Experiment  Station, 
Vol.  VII)  in  which,  from  many  analyses  of  sludge  in  England 
and  Germany,  the  amount  of  dried  material  in  sewage  was 
established  at  2.1  oz.  (60  g.)  per  capita  per  day,  when  including 
storm  water  but  not  industrial  wastes.  This  value  gives,  with 
an  amount  of  water  in  the  sludge  of: 

95  per  cent.  2.1X20  =  42  oz.  (0.06X20  =  1.2  1.)  sludge  per 
capita  per  day. 

90  per  cent.  2.1x10  =  21  oz.  (0.06X10  =  0.6  1.)  sludge  per 
capita  per  day. 

80  per  cent.  2.1x5  =  10.5  oz.  (0.06X5  =  0.3  1.)  sludge  per 
capita  per  day. 

We  may  approximate  the  same  result  by  assuming  an  amount 
of  suspened  matter  of  17.5  to  35  grains  per  gallon  (300  to  600 
mg.  per  1.),  as  is  the  rule  with  city  sewage  in  Germany.  With  a 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     17 


specific  gravity  of  1.1  we  obtain  12.6  to  25.2  cu.  in.  per  cubic  yard 
(0.27  to  0.54 1.  per  cbm.)  of  dried  material.  As  the  larger  floating 
substances  are  not  included  in  the  analyses  this  figure  should  be 
somewhat  greater,  so  that  14  to  28  cu.  in.  per  cubic  yard  (.3  to  .6 
1.  per  cbm.)  of  sludge  containing  90  per  cent,  water  may  be 
expected. 

In  estimating  these  values  no  account  is  taken  of  a  partial 
elimination  of  the  solid  matter  in  the  clarification  plant.  The 
efficiency  of  the  method  of  clarification  is  therefore  to  be  taken 
into  account. 

These  methods  of  estimating  are  valueless,  possibly  in  the  case 
of  grit  chambers,  but  particularly  with  screening  or  contact  bed 
treatment.  Here  one  must  trust  to  experience  and  experiment. 
It  is  in  any  case  of  greater  value  to  consider  experience  with  the 
plants  of  towns  where  the  conditions  are  similar  than  to  depend 
upon  theoretical  calculations  of  the  sludge  to  be  expected. 

AMOUNT  OF  DETRITUS  FROM  GRIT  CHAMBERS  AND 
SCREENS 


American 

measures 

Metric  n 

leasures 

Place 

Cu.  yds. 
per  million 
gallons 
sewage 

Cu.  yds. 
per  1000 
inhabi- 
tants per 
day 

Liters  per 
1000  cubic 
meters 
sewage 

Liters  per 
1000  in- 
habitants 
per  day 

Method 
employed 

Leipzig 

0  067 

• 

13  5 

Coarse    screen- 

Charlottenburg 

0  014 

11 

ing. 
Grit     chamber 

Hamburg  
Ohrdruf  

0.826 

0.056 
0.079 

167 

43 
60 

and  screening. 
Grit     chamber 
and  screening. 
Grit      chamber 

Schoneberg  

Marburg  
Marburg  
Frankfort-on-Main  
Frankfort-on-Maia  
Cologne 

0.643 

0.643 
0.643 
0  356 

0.021 

0.027 
0.022 
0.038 
0.038 

130 

130 
130 

72 

16 

21 

17 
29 
29 

and  screening. 
Grit     chamber 
and  screening. 
Grit  chamber. 
Screening. 
Grit  chamber. 
Screening. 
Grit  chamber. 

Cologne 

0  905 

183 

Screening. 

Elberfeld 

0  426 

0  025 

86 

19 

Grit  chamber. 

Elberfeld  
Hanover.    . 

0.639 
1.069 

0.037 
0  .  033 

129 
216 

28 
25 

Screening. 
Grit  chamber. 

Dresden  

0.149 

0.007 

30 

5 

Grit  chamber. 

Dresden  
Munich-Gladbach  

0.500 
0.371 

0.022 
0.076 

101 
75 

17 

58 

Riensch  disc. 
Screening. 

18 


SEWAGE  SLUDGE 


The  following  tables  give  some  results  of  the  amounts  of  sludge 
obtained  by  various  processes.  The  figures  are  obtained  partly 
from  reports  and  partly  by  direct  information  furnished  by  city 
authorities. 

The  amounts  may  be  estimated  per  million  gallons  of  sewage 
or  per  capita  per  day.  The  first  method  is  used  in  towns  where 
much  of  the  sewage  comes  from  manufacturing  concerns,  while 
the  latter  affords  a  better  comparison  where  these  are  absent, 
because  the  different  volumes  of  water  used  are  eliminated. 

It  is  further  to  be  noted  that  in  estimating  the  separate  quanti- 
ties the  volume  of  sludge  is  usually  ascertained  quite  accurately 
by  the  mark  of  the  surface  level  in  the  tank,  by  the  contents  of 
the  vacuum  receiver  or  the  sludge-press,  or  that  of  the  drying 
bed,  while,  on  the  contrary,  the  volume  of  the  clarified  sewage  in 
a  specified  period,  especially  in  towns  with  combined  sewerage, 
is  usually  not  closely  enough  ascertained. 

The  results  with  these  methods  naturally  vary  greatly,  especi- 
ally with  bar  screens  and  mesh  screens,  on  account  of  the  differ- 
ence in  the  size  of  the  mesh  and  the  spacing  of  the  bars;  but  this 
is  so  with  regard  to  grit  chambers  also,  although  in  less  degree, 
on  account  of  the  different  velocities  and  other  reasons  already 
given. 

AMOUNT  OF  SLUDGE  FROM  SEDIMENTATION  TANKS 


American 

measures 

Metric  n 

tieasures 

Place 

Cu.  yds.  per 
million 
gallons 
sewage 

Cu.  yds.  per 
1000  per- 
sons day 

Liters  per 
cubic  me- 
ter of 
sewage 

Liters  per 
capita  per 
day 

Method  of 
clarification 

Frankfort 

16.3 

0.930 

3.3 

0.71 

Tanks. 

Bremen 

10.9 

0.655 

2.2 

0.50 

Tanks. 

Hanover 

9.9 

0.301 

2.0 

0.23 

Tanks. 

Mannheim  
Munich-Gladbach  
Cassel  
Brieg 

10.9 
12.4 
23.8 
17  8 

0.877 
2.620 
0.628 
0  498 

2.2 
2.5 
4.8 
3  6 

0.67 
2.00 
0.48 
0.38 

Tanks. 
Tanks. 
Tanks,  i 
Wells. 

Stargard 

37  1 

0  589 

7  5 

0.45 

Wells. 

Culmsee  
Langensalza  
Leipzig  

Leipzig 

123.8 
123.8 
23.8 

0  8 

0.877 
2.188 

25. 
25. 
4.81 

0  16 

0.67 
1.67 

Tanks. 
Wells. 
Primary   con- 
tact beds. 
Secondary  con- 

Failswerth   

2.8 

0.092 

0.56 

0.07 

tact  beds. 
Secondary  con- 
tact beds. 

1  Without  grit  chambers  and  screens. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     19 


With  grit  chambers  we  estimate  0.37  to  0.75  (average  0.50) 
cu.  yds.  per  million  gallons  [75  to  150  (average  100)  1.  per  1000 
cbm.]  of  sewage  and  possibly  0.013  to  0.026  cu.  yds.  (10  to  20 
1.)  per  100  inhabitants  daily.  The  same  figures  are  to  be  used 
for  bar  screens.  They  are  of  course  not  correct  for  simple  coarse 
bar  screens  which  are  only  intended  to  keep  coarse  material  from 
the  pumps  or  plant.  The  amounts  for  these  are  much  less, 
although  with  large  screening  plants  they  increase  somewhat. 

In  the  larger  German  plants  the  amount  of  sludge  is  fairly 
uniform.  It  is  about  10  to  25  cu.  yds.  per  million  gallons  (2  to 
5  1.  per  cbm.)  or  0.39  to  1.31  cu.  yds.  per  1000  persons  (0.3  to 
1.0  1.  per  capita)  per  day.  These  differences  result  from  the 
causes  already  mentioned,  but  above  all  from  the  water  con- 
tained in  the  sludge.  Therefore  the  places  mentioned  in  the 
tables  employing  clarification  by  wells  have  comparatively  large 
amounts  of  sludge.  For  example,  the  high  figures  for  Langen- 
salza  result  from  the  continuous  removal  of  fresh  sludge.  In 
Culmsee,  on  the  other  hand,  the  separate  system  exists  and  the 
sewage  is  fairly  concentrated.  Preliminary  treatment  by  con- 
tact beds  gives  values  similar  to  sedimentation,  special  weight 
being  laid  upon  the  greatest  possible  previous  removal  of  sus- 
pended matter  in  order  to  protect  the  beds.  Subsequent  treat- 
ment, which  is  only  used  with  sprinkling  filters,  on  account  of  the 
particles  of  deposit  frequently  washed  out,  naturally  shows  but 
a  small  amount  of  sludge. 

AMOUNT  OF  SLUDGE  FROM  SEPTIC  TANKS 


American 

measures 

Metric  r 

neasures 

Place 

Cubic  yards 
per  million 
gallons 
sewage 

Cubic  yards  per 
1000    persons 
daily 

Liters   per 
cubic  meter  of 
sewage 

Liters  per 
capita     per 
day 

Manchester  .... 

12  0 

0.62 

2  43 

0.47 

Accrington  
Colne  

7.9 
7.4 

0.18 

1.6 
1.5 

0.135 

Hampton  
Birmingham  
Stuttgart  
Merseburg  
Mullheim  
Unna  
Leipzig1.  . 

4.36 
16.85 
18.6 
8.2 
13.8 
9.9 
7.3 

0.50 
0.10 
0.46 
0.26 

0.88 
3.4 
3.75 
1.65 
2.0 
2.0 
1.47 

0.38 
0.08 
0.35. 
0.20 

Emscherbrunnen  
Halberstadt  

47.5 

0.13-0.33 
1.26 

9.6 

0.1-0.25 
0.96 

Preliminary  biological  treatment  in  experimental  plant. 


20 


SEWAGE  SLUDGE 


With  septic  tanks  the  amount  of  sludge  produced  is  less  the 
more  complete  the  septic  action  (Merseburg) .  Where  the  sewage 
is  chiefly  domestic  and  without  storm  water,  which  always  adds 
much  mineral  matter,  the  amount  is  about  0.13  to  0.26  cu.  yds. 
for  each  1000  persons  (0.1  to  0.2  1.  per  capita)  daily,  or  7.4  to  12.4 
cu.  yds.  per  million  gallons  (1.5  to  2.5  1.  per  cbm.)  of  sewage.  In 
the  other  case,  as  well  as  in  the  presence  of  much  trade  waste,  the 
limit  is  increased  to  about  0.39  cu.  yds.  per  1000  persons  (0.3  1. 
per  capita)  per  day,  or  to  17.3  cu.  yds.  per  million  gallons  (3.5  1. 
per  cbm.).  Therefore  many  of  the  English  cities  show  much 

AMOUNT  OF  SLUDGE  FROM  CHEMICAL  PRECIPITATION 


American 

measures 

Metric  ir 

easures 

Place 

Cubic  yards 
per  million 
gallons 
sewage 

Cubic  yards 
per  1000 
persons 
daily 

Liters  per 
cubic  meter 
of  sewage 

Liters  per 
capita  per 
day 

Precipitant 

Chorley  
Hendon  
Lichfield 

79 
69 
29.7 

3.64 
2.98 

16 

14 
6 

2.78 
2.27 

Aluminoferric 
Aluminoferric 
Aluminoferric 

Sheffield  
Bury  
Guildford  
Glasgow  

London  

Leipzig  

Essen  .  .  . 

13.4 
74 
111.9 

42 

32.2 

24.1 
19.8 

0.54 

2.19 
4.01 

1.66 
.80 

2.7 
15 
22.6 
8.5 

6.5 

4.87 
4.00 

0.41 
1.67 
3.06 

1.27 
0.61 

Lime 
Aluminoferric 
Aluminoferric 
Lime  and  sulphate  of 
alumina 
Lime  and  sulphate  of 
iron 
Oxide  of  iron 
Lime 

larger  amounts  of  sludge.  In  Halberstadt  the  high  figure  is 
attributable  to  the  limited  amount  of  digestion  as  the  sludge  is 
removed  here  every  8  weeks. 

Chemical  treatment  produces  a  varied  amount  of  sludge  de- 
pending upon  the  process  employed  and  the  amount  of  pre- 
cipitant used,  as  the  latter  produces  a  large  volume  of  sludge 
containing  much  water.  Thus,  1.71  grains  of  lime  per  gallon 
(100  g.  per  cbm.)  of  sewage,  if  the  precipitation  were  complete, 
would  give  2.2  Ibs.  (1000  g.)  of  sludge  with  90  per  cent,  moisture. 
But  often  .44  Ib.  (200  g.)  to  even  1.1  Ibs.  (500  g.)  of  precipitant 
are  added  separately.  The  increased  clarification  effected  by 
sedimentation,  possibly  75  to  80  per  cent,  of  the  suspended  matter 
as  compared  with  60  to  70  per  cent,  by  mechanical  methods, 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     21 

results  jn  larger  deposits  of  sludge.  In  general  the  amount  is 
25  to  50  cu.  yds.  per  million  gallons  (5  to  10 1.  per  cbm.)  of  sewage. 
The  low  efficiency  observed  at  Sheffield  is  explained  by  the  fact 
that  one-third  of  the  sewage  there  is  wash  water. 

The  lignite  treatment,  which  is  closely  allied  to  chemical  treat- 
ment, removes  yet  greater  amounts  of  sludge,  100  to  125  cu.  yds. 
per  million  gallons  (20  to  25  1.  per  cbm.)  of  sewage,  partly  because 
a  very  watery  sludge  is  obtained,  partly  because  the  lignite  and 
sulphate  of  alumina  increase  the  volume  greatly  by  the  absorp- 
tion of  water.  At  Copenick  on  the  other  hand,  where  the  sludge 
is  obtained  by  drying  out  in  the  tanks  to  about  60  per  cent, 
moisture,  they  get  12. 5  cu.  yds.  per  million  gallons  (2.5  1.  per  cbm.), 
or  1.18  cu.  yds.  per  1000  inhabitants  (0.9  1.  per  capita)  daily. 

From  the  data  given  the  amount  of  sludge  to  be  expected  may 
(in  spite  of  some  considerable  differences)  be  estimated  with 
sufficient  accuracy  for  a  given  size  of  plant  and  the  arrangements 
for  its  treatment  and  utilization. 

Chemical  treatment  gives  the  greatest  amount  of  sludge  and 
has,  therefore,  been  abandoned  in  many  cases,  while  the  septic 
tank  gives  the  least.  The  rule  applies  to  all  methods,  that  the 
more  thorough  the  purification  the  greater  the  resulting  amount 
of  sludge. 


CHAPTER  III 
THE  REMOVAL  OF  SLUDGE  FROM  CLARIFICATION  TANKS 

A  well  considered  plan  for  removing  the  sludge  from  the 
tank  and  for  its  transport  to  places  where  further  drying  or 
treatment  is  carried  on,  is  of  great  importance;  for  upon  this 
rests  the  efficiency  of  various  methods  of  treatment.  Moreover, 
foul  odors  may  in  this  way  be  minimized,  if  not  entirely  avoided, 
and  large  sums  of  money  saved,  especially  in  wages. 

The  plants  and  their  details,  therefore,  vary  very  greatly  and 
in  general  must  be  adapted  to  the  methods  of  cleaning  and  of 
operation  as  well  as  to  the  topographical  conditions. 

The  general  principles  to  be  observed  in  the  treatment  of 
sewage  may  be  summarized  as  follows: 

1.  The  more  frequently  sludge  has  to  be  removed  and  the 
larger  its  amount,  the  greater  the  attention  that  must  be  given 
to  the  matter  and  the  greater  the  cost  in  any  given  case. 

2.  The  condition  of  the  sludge  which  is  favorable  for  its  later 
use  must  not  be  altered  to  its  detriment  in  removal. 

3.  The  removal  of  sludge  must  be  accomplished  with  the  least 
possible  work. 

4.  This  should  be  effected,  so  far  as  possible,  without  manual 
labor. 

5.  The  removal  should  be  as  complete 'as  possible. 

6.  With  mechanical  appliances  it  is  important  that  all  parts 
should  be  as  simple  as  possible,  especially  movable  parts,  and 
that  their  location  should  be  above  water. 

Referring  to  1,  the  periods  of  cleaning  are  largely  dependent 
on  the  method  employed. 

The  removal  of  detritus  from  grit  chambers  usually  takes 
place  after  the  receptacles  provided  for  it  are  filled,  in  order  to 
avoid  encroaching  on  the  waterway  and  preventing  insufficient 
sedimentation.  A  daily  cleaning  is  only  required  where  the 
amount  removed  is  very  large,  as  otherwise  excessive  dimensions 
for  the  grit  chamber  would  be  necessary,  and  this  removal  should 
then  be  by  mechanical  means. 

22 


TREATMENT  AND   UTILIZATION  OF  SLUDGE     23 

With  mesh  screens  and  bar  screens  the  detritus  should,  natur- 
ally, be  constantly  removed.  Here,  too,  the  removal  in  large 
plants  is  mechanical  and  automatic. 

The  intervals  between  the  removal  of  sludge  from  semimenta- 
tion  tanks  depend  on  the  putrefaction  of  the  material  contained. 
This  is  controlled  by  the  nature  of  the  sewage  and  also  by  the 
temperature.  Therefore  the  sludge  may  remain  in  the  tanks  3 
to  7  days  in  summer,  8  to  12  in  winter.  In  chemical  treatment 
these  periods  can  be  increased,  especially  when  this  method 
interferes  with  putrefaction.  At  Leipzig,  e.g.,  the  sludge  is  drawn 
off  every  10  to  20  days. 

In  septic  tanks  the  sludge  should  be  removed  at  much  longer 
intervals.  Its  storage  is  here  the  important  matter.  With 
small  installations  removal  need  only  take  place  from  one  to 
two  times  a  year,  in  larger  ones  very  one  to  three  months.  In 
general,  an  infrequent  removal  should  be  aimed  at,  as  only  in 
this  way  can  the  advantage  of  the  septic  tank  be  fully  realized; 
for  the  amount  is  diminished  by  long  storage,  as  has  already 
been  mentioned,  and  it  also  becomes  less  offensive.  In  any  case 
an  increase  of  suspended  matter  in  the  effluent  caused  by  the 
accumulation  of  deposit  and  a  resulting  increase  of  velocity, 
indicate  the  time  for  cleaning.  Another  method,  and  one  to  be 
recommended  in  large  plants  with  large  volumes  of  sludge,  is  to 
draw  off  a  part  at  shorter  intervals  during  operation;  while  the 
main  cleaning  should  be  done  in  the  fall  and  spring,  especially 
where  it  is  used  as  a  fertilizer.  When  the  removal  is  made,  as 
at  Leeds,  through  plug  valves  at  the  bottom,  the  result  is  the 
thickest  and  most  thoroughly  digested  sludge  and  an  objection- 
able interference  with  its  free  flow  is  prevented.  Moreover,  the 
tank  is  in  continuous  use  and  the  sludge  chamber  can  accommo- 
date smaller  volumes  (Emscher  tanks).  - 

The  intervals  between  cleanings  in  contact  beds  are  so  varied, 
according  to  their  construction  and  demands,  that  no  figures 
can  be  given. 

The  amounts  of  sludge  from  the  different  processes  are  given 
in  the  previous  chapter. 

As  to  2,  the  favorable  condition  of  the  sludge,  especially  the 
small  amount  of  water  contained,  must  not  be  altered  in  its 
removal.  If  sludge  from  septic  tanks,  for  instance,  must  be 
stirred  up  so  as  to  be  removed  by  pumps,  the  proportion  of 
water,  and  consequently  the  entire  volume,  is  increased,  although 


24  SEWAGE  SLUDGE 

the  water  is  given  off  again  more  readily  on  account  of  its  com- 
position than  in  plain  sedimentation.  In  such  cases  sludge  from 
the  combined  system  leads  to  better  results  by  drawing  off  the 
upper  dilute  layer  by  pumps,  while  the  lower,  settled  layers  are 
excavated,  as  in  Unna.  Sludge  from  plain  sedimentation  also 
contains  more  moisture  where  the  stirring  process  or  flushing 
pipes  are  used. 

With  the  Kremer  apparatus,  where  the  sludge  of  the  bottom 
layer  contains  but  85  per  cent,  of  water  or  less  (as  here  the  grease 
and  cellulose  particles  which  attract  a  large  amount  of  moisture 
are  removed),  this  preservation  of  the  favorable  consistency  of 
the  sludge  in  its  removal  is  most  important. 

Where  the  fine  greasy  material  is  removed  separately  by  being 
passed  through  several  tanks,  the  ability  to  keep  it  separate  is 
desirable  in  order  to  work  it  over  into  grease,  or  else  to  bury  it 
wet,  while  the  coarser  portion  can  be  dried. 

3.  The  work    of  disposing  of  sludge    can  often  be    greatly 
lightened  by  an  intelligent  use  of  the  land.     The  places  for  drying 
sludge  should  therefore  be  located  as  low  as  possible  in  order  to 
permit  of  its  discharge  from  the  tanks  by  gravity.     This   is 
especially  desirable  where  the  tanks  are  elevated  above  the  sur- 
rounding land  on  account  of  an  unfavorable  soil  for  foundation 
and  ground  water   (Rheydt)   or  where  the  entire  plant  is  on 
sloping  ground  (Siegen). 

Much  may  also  be  gained  by  a  favorable  arrangement  of  the 
ground  plan  of  the  separate  parts,  such  as  tanks,  sludge  wells, 
pumps  and  beds  and  apparatus  for  drying  sludge. 

As  sludge  containing  much  sand  does  not  flow  so  readily  and 
therefore  demands  a  steeper  slope  in  the  bottom  of  the  tank 
and  in  the  pipes,  it  is  advisable  to  place  a  good  grit  chamber  for 
its  interception.  The  suction  of  the  pumps  will  then  be  more 
efficient  and  erosion  lessened. 

Much  labor  can  be  saved  by  forethought  in  designing  the 
tanks  for  the  removal  of  sludge.  The  use  of  mechanical  appara- 
tus, such  as  will  be  described  later,  will  be  of  advantage  in 
certain  cases. 

4.  In  spite  of  the  large  numbers  of  bacteria  in  sludge  (over 
11,000,000  per  c.c.  were  found  in  one  English  examination  of 
fresh  sludge)  the  number  of  pathogenic  germs  is  small.     There- 
fore there  is  but  little  illness  among  the  employees  of  these  plants 
that  can  be  traced  to  them.     There  is,  however,  some  possibility 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     25 

that  the  germs  of  contagious  diseases  in  a  city  may  exist  in  the 
sewage  and  sludge  in  large  numbers  until  they  are  sufficiently 
disinfected. 

For  this  reason,  and  because  the  exhalation  from  the  sludge  as 
it  putrefies,  as  it  almost  always  does  in  summer,  are  detrimental 
to  the  health  of  the  workmen,  their  personal  contact  with  the 
sludge  should  be  avoided.  The  high  wages  required  to  get  this 
dirty  and  unpopular  work  done  can  also  thus  be  saved. 

5.  When  portions  of  putrescent  material  remain  in  the  tanks 
after  the  removal  of  sludge,  the  fresh  sewage  entering  becomes 
contaminated  and  in  a  few  hours  septic.    The  bubbles  of  gas  which 
form  in  septic  tanks  cause  particles  of  sludge  to  rise  and  then  pass 
out  in  the  effluent  or  adhere  to  the  contact  beds,  which  may  be 
inserted   subsequently,   hastening  the  process  of  sludging.     In 
this  connection  all  devices  for  drawing  off  the  sludge  from  below 
the  surface,  and  whose  proper  operation,  therefore,  cannot  be 
observed,  should  receive  careful  supervision. 

Formerly  but  little  weight  was  attached  to  this  matter  in 
England,  and  as  the  outflowing  liquid  became  putrescent  and 
carried  off  larger  flakes  of  suspended  matter,  plain  sedimentation 
tanks  were  abandoned  in  favor  of  chemical  precipitation. 

If  particles  of  sludge  remain  for  any  length  of  time  in  the  tank 
they  become  compacted  and  their  loosening  and  removal,  per- 
haps by  flushing  nozzles  or  by  stirring,  always  becomes  more 
difficult. 

6.  It  is  unnecessary  to  point  out  that  the  principles  applicable 
to  all  hydraulic  work — that  of  providing  mechanical  details  of 
the  greatest  simplicity  and  strength  and  locating  their  moving 
parts  so  far  as  practicable  above  water — are  especially  important 
in  dealing  with  sewage  and  sludge.     One  favorable  quality  in 
sewage  is  to  be  noted:  that  the  contained  grease  acts  in  many 
cases  as  a  lubricant  on  the  moving  parts  with  which  it  comes  in 
contact. 

REMOVAL  OF  DETRITUS  FROM  GRIT  CHAMBERS 

Detritus  is  most  frequently  removed  by  suspending  operation 
and  cleaning  it  out  with  a  shovel  by  hand  after  the  sewage  has 
been  pumped  off.  For  this  purpose  it  is  necessary  to  divide  the 
grit  chamber.  A  further  division  is  sometimes  made  in  the  effort 
to  maintain  the  most  favorable  velocity  for  depositing  the  mineral 


26 


SEWAGE  SLUDGE 


matter  by  changing  the  flow  of  sewage  by  operating  or  cutting- 
out  the  different  units. 

In  smaller  plants  the  deposited  material  is  often  collected  in 
buckets  set  in  a  steeply  sloping  pit  and  these  are  lifted  out  by 
cranes.  Larger  plants  are  often  provided  with  bucket  dredges. 
These  can  be  stationary  provided  the  bottom  has  a  steep  slope 
(1:1)  toward  the  dredge,  but  the  movement  in  a  vertical  direction 
must  be  sufficiently  great  for  it  to  be  taken  entirely  out  of  the 
sewage  after  being  used.  This  is  necessary  to  avoid  obstruction 
to  the  current  of  the  sewage  and  to  prevent  the  rusting  of  the 
bearings.  In  order  to  provide  a  steeper  slope  to  the  end  walls 
with  a  greater  length  to  the  grit  chamber  it  is  advisable  to  pass 
the  chain  for  the  buckets  over  two  pulleys,  as  in  Figs.  1  and  2,  as 
is  done  in  Manchester.  With  a  shallow  pit  the  dredge  must  move 
horizontally.  Sharp  angles  between  bottom  and  side  walls 
should  be  avoided,  as  these  wrill  not  be  reached  by  the  dredge  and 
facilitate  deposits  of  decomposing  sludge.  Cleaning  may  then 
take  place  during  operation,  as  at  Frankfort  and  Elberfeld. 
With  very  fine  sand  this  may  cause.trouble,  as  it  will  be  stirred  up 


FIG.  1.  FIG.  2. 

FIGS.  1  and  2. — Arrangement  for  cleaning  grit  chamber. 

and  washed  out  by  the  dredging.  For  this  reason  the  dredge 
was  abandoned  at  Marburg.  In  place  of  the  bucket-and-chain 
dredge  a  clam-shell  dredge  may  be  used,  but,  in  order  to  prevent 
injury  to  the  bottom,  only  when  the  plant  is  not  in  operation. 

On  account  of  the  depth  of  detritus  in  grit  chambers,  its 
removal  by  pumps  or  steam  ejectors,  as  was  attempted  at  Diissel- 
dorf,  is  not  feasible,  as  the  sand  is  mixed  with  water  which  then 
has  to  be  again  separated.  At  Merseburg,  on  the  other  hand,  a 
Wegner  patent  portable  suction  pump  was  successfully  employed, 
which  will  be  spoken  of  later. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     27 

The  mechanical  devices  for  the  removal  of  detritus  from  mesh 
screens  and  bar  screens,  which  are  usually  an  intrinsic  part  of 
the  plant,  will  not  be  particularly  mentioned  here.  Further 
particulars  may  be  found  in  Dunbar's  "Principles  of  Sewage 
Treatment"1  and  Schmeitzner's  "Clarification  of  Sewage.7'2  In 
smaller  plants  the  cleaning  is  usually  done  by  hand  with  rake  or 
spade. 

Contact  beds  are  taken  apart  for  cleaning,  and  the  material 
freed  from  deposit  by  rinsing  or  washing  by  hand  or  by  machines, 
such  as  are  used  for  gravel  filters. 

REMOVAL  OF  SLUDGE  FROM  TANKS,  WELLS  AND  TOWERS 

The  question  of  removing  sludge  from  tanks,  wells  and  towers 
in  sedimentation  plants  or  from  chemical  precipitation  tanks  is 
of  great  importance.  It  is  a  question  involving  much  larger 
volumes  and  also  a  more  frequent  removal  made  necessary  by 
these  methods  of  clarification. 

We  may  make  a  distinction  at  this  point  between  (a)  removal 
with  interruption  of  operation,  and  (b)  removal  during  operation. 

a.  REMOVAL  WITH  INTERRUPTION  OF  OPERATION 

In  this  method  the  tanks  are  allowed  to  remain  quiet — for 
tanks  are  almost  always  used  in  this  method — for  1  or  2  hours 
after  cutting  off  the  supply.  The  clarified  liquid  above  the 
sludge  is  then  discharged  into  the  outfall  through  an  outlet  con- 
trolled by  gates  or  stop-planks.  With  a  fixed  overflow  weir 
there  is  sometimes  a  special  by-pass  channel  with  a  controlling 
valve.  The  turbid  liquid  which  then  remains  in  the  tank  above 
the  sludge  must  usually  be  drawn  off  by  pumps  or  a  vacuum 
receiver,  and  conveyed  to  the  influent  conduit  for  a  second  clari- 
fication. Where  there  are  several  tanks  it  can  be  brought 
through  a  sufficiently  deep  connecting  channel  to  a  clean  empty 
tank,  in  this  way  saving  some  cost.  This  tank  is  then  filled  with 
unsettled  sewage  and  the  process  continued. 

The  drawing  off  of  the  turbid  liquid  must  be  done  in  such  a  way 
that  it  is  removed  as  completely  as  possible  down  to  the  under- 
lying layer  of  sludge  without  stirring  this  up.  For  this  purpose  a 

1  "Clarification  of  Sewage,"  by  Dr.  Ing.  Rudolph  Schmeitzner.     Translated  by  A.  Elliott 
Kimberly.     Eng.  News  Pub.  Co.,  N.  Y.,  1910. 

2  "Principles  of  Sewage  Treatment,"  by  Prof.  Dr.  Dunbar.     Translated  by  H.  T.  Calvert. 
J.  B.  Lippincott  Co.,  Phila.,  1908. 


28 


SEWAGE  SLUDGE 


movable  weir,  of  which  there  are  various  types,  may  be  used,  or 
channels  at  different  levels,  such  as  are  patented  by  the  firm  of 
Geiger  in  Karlsruhe,  and  have  been  furnished  by  them  for  Elber- 
feld.  Here  there  is  a  drum  whose  casing  is  perforated  by  short 
spiral  slits  placed  behind  an  iron  plate  with  a  vertical  slit  set  in 
the  wall  of  the  tank.  The  liquid  is  gradually  drawn  off  to  lower 
levels  by  the  rotation  of  the  drum  containing  the  slits  which 
overlap  those  in  the  plate  at  different  elevations. 

The  same  result  is  secured  at  Munich-Gladbach  by  a  pipe 
which  can  be  telescoped.  At  the  upper  end  this  has  been  en- 
larged like  a  funnel  to  secure  a  broad  overflow  and  so  avoid 
uneven  disturbances. 

At  Copenick  the  emptying  of  earth  tanks  having  a  capacity 
of  about  3,440,000  gallons  (1300  cbm.),  is  accomplished  every 
3  to  4  weeks  in  2  days  by  8  pipes  of  5.85  in.  (15  cm.)  clear 
diameter  placed  at  different  elevations.  These  lie  in  a  wall  9  ft. 


FIG.  3. — Floating  arm  for  drawing  off  supernatant  liquid. 

10  in.  (3  m.)  long,  which  also  serves  as  an  overflow  weir,  and  are 
closed  by  iron  flap  valves  set  at  the  ends  on  the  water  side  at  an 
angle  of  45  degrees.  These  are  closed  by  the  pressure  of  the 
sewage  and  can  be  opened  by  chains  from  above.  This  simple 
device  has  proved  very  efficient. 

All  these  arrangements,  however,  necessitate  careful  watching 
during  operation.     This  is  rendered  unnecessary  by  the  floating 
arm  devices  which  were  first  used  in  England. 

Here  there  is  a  circular  or  square  pipe  attached  to  a  fixed 
horizontal  one,  which  can  be  swung  in  a  vertical  plane  (Fig.  3). 
The  upper  end  is  always  kept  8  to  10  in.  (20  to  25  cm.)  below 
the  surface  of  the  sewage  by  one  or  two  floats.  Consequently 
the  sewage  is  only  drawn  off  from  the  top  layer.  At  the  same 
time  the  opening  being  submerged  prevents  floating  substances, 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     29 

such  as  scum  and  grease,  from  flowing  off.  The  same  purpose  is 
sometimes  served  by  a  protecting  box  fastened  between  two 
floats  (Fig.  4)  or  a  floating  scum  board  which  cuts  off  a  portion 
of  the  tank  in  which  the  floating  arm  is  located  and  which  moves 
in  two  grooves  in  the  sides  of  the  tank. 

A  valve  is  inserted  in  the  horizontal  pipe  to  regulate  the  dis- 
charge. In  order  to  draw  the  sewage  from  the  tank  more  evenly, 
the  pipe  leading  to  the  valve  may  be  divided  into  two  branches 
which  drop  to  each  side  of  the  tank  (Fig.  4) . 


//MfM^^ 

FIG.  4. — Double  floating  arm. 

In  employing  a  vacuum  receiver  for  the  removal  of  the  turbid 
liquor  the  introduction  of  a  well  for  this  liquid  is  advisable,  as 
this  renders  the  effluent  independent  of  the  intermittent  opera- 
tion of  the  apparatus. 

In  wells  where  there  is  not  room  for  such  a  floating  arm,  which 
must  be  longer  owing  to  the  greater  depth,  a  hose  may  be  used 
which  can  be  lowered  by  a  chain  as  the  sewage  is  drawn  down 
(Harburg),  or  the  suction  end  of  which  is  kept  submerged  by 
floats.  In  general  there  will  be  found  no  necessity  for  special 
devices  in  wells  for  the  removal  of  the  turbid  liquor  but  the 
sludge  pipes  can  be  used  for  repairs  and  inspection  purposes. 

After  the  roily  sewage  has  been  removed  the  sludge  must  be 
drawn  off.  For  this  purpose  a  sludge  sump  should  be  provided 
in  the  tank,  from  which  the  pump  draws  off  the  sludge  directly, 
or  a  sludge  well  should  be  inserted.  The  best  position  for  the  sump 
in  tanks  with  the  ordinary  inclination  is  directly  after  the  inlet, 
as  most  of  the  sludge  is  deposited  here.  In  the  experiments  at 


30  SEWAGE  SLUDGE 

Cologne  with  a  velocity  of  1.56  in.  (40  mm.)  per  second  in  the 
1.1  ft.  (3.35  m.)  long  sump  (which  is  placed  at  the  inlet  just  in  front 
of  the  regulating  device  for  securing  a  uniform  distribution  of 
flow),  about  45  per  cent,  of  the  sludge  was  deposited,  while  in  the 
remaining  length  of  the  tank — fully  130  ft.  (40  m.) — only  55 
per  cent,  was  deposited.  With  a  velocity  of  0.78  in.  (20  mm.) 
the  proportion  was  51  per  cent,  to  49  per  cent.,  and  with  a  veloc- 
ity of  0.156  in.  (4  mm.),  70.7  per  cent,  in  the  sludge  sump  and 
29.3  at  the  bottom  of  the  tank  (Fig.  5).  At  the  same  time  such 
a  location  of  the  sump  prevents  a  silting-up  of  the  cross-section 
and  gives  to  the  bottom  of  the  tank  an  inclination  toward  the 
inlet  favorable  to  effective  sedimentation.  On  account  of  the 
opposing  current  of  the  sewage,  however,  the  flow  of  the  sludge 
is  retarded 

4ffrvm  20mm  ¥mm 

29,3% 


S/%  I    70,7% 

FIG.  5. — Deposition  of  grit  in  grit  chambers. 

While  it  was  formerly  thought  to  be  advisable  in  constructing 
sedimentation  tanks  to  place  baffle  walls  and  other  impediments 
to  the  flow  of  the  sewage  in  order  to  promote  clarification,  more 
recent  methods  aim  rather  at  preventing  detrimental  currents 
and  at  removing  the  sludge  as  simply  and  economically  as 
possible.  For  this  reason  the  ground  plan  should  be  regular  in 
shape  and  above  all  sharp  angles  and  corners  should  be  avoided 
from  which  it  may  be  difficult  to  remove  the  sludge. 

Earthen  tanks  are  not  advisable  for  thorough  sedimentation, 
as  they  require  frequent  cleaning  and,  even  for  experimental 
plants  a  lining  of  cement  or  planks  (which  have  been  found 
serviceable  in  Bremen)  should  be  employed.  Otherwise  the 
clarification  tank  must  be  used  for  sludge  drying  as  well,  as  the 
muddy  bottom  would  be  removed  with  the  liquid,  sludge.  In 
drying  sludge  in  tanks,  too,  it  is  well  that  the  sub-soil  should 
have  some  marked  characteristic,  such  as  a  light  color.  Above 
all,  it  is  impracticable  to  provide  a  tank  with  a  natural  bottom 
of  sufficient  slope  to  convey  the  sludge  readily  to  the  sump. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    31 

This  mere  detail  is  of  particular  value  in  tanks,  for  it  is  a 
disadvantage  in  this  type  of  clarification  chamber  that  the 
sludge  must  be  removed  separately  from  a  large  surface,  and  in 
cleaning  by  hand  must  be  pushed  to  the  sump  by  wooden  or 
rubber  covered  scrapers.  With  the  frequent  cleaning  necessary 
in  sedimentation  tanks  this  results  in  a  great  deal  of  labor  and 
expense.  It  is,  moreover,  harmful  to  the  workmen,  who  must 
often  wade  up  to  the  knees  in  sludge  and  inhale  the  noxious  gases 
from  the  decomposing  material. 

The  attempt  is  therefore  made  to  so  construct  the  bottom  of 
the  tanks  that  the  sludge,  in  pumping,  will  always  flow  by  gravity 
to  the  pump  well. 

The  slope  in  general  use,  say  1:100  (Mannheim  and  Cassel) 
to  1:45  (Hanover)  is  not  sufficient,  for  experience  has  shown 
that  some  aid  by  manual  labor  cannot  be  dispensed  with  in 
these  tanks. 

For  an  easy,  automatic  flow  with  settled  sludge  containing  at 
least  90  per  cent,  of  water  a  slope  of  1:  10  to  1:15  is  necessary, 
depending  on  whether  there  is  much  sand  and  coarse  material, 
or  whether  there  is  a  fine,  fluid  sludge.  Such  a  steep  slope  is 
not  feasible  with  tanks  130  ft.  (40  m.)  long.  In  some  tanks  a 
channel  for  the  sludge  has  been  built  in  the  bottom,  which  grad- 
ually increases  in  depth  and,  for  example,  in  Mannheim  with  an 
inclination  of  the  bottom  of  1:100,  is  given  a  fall  of  1:  50,  in 
Munich-Gladbach  one  of  1:25.  The  attempt  has  been  made, 
in  addition  to  increasing  the  fall,  to  reduce  the  friction  of  the 
sludge  in  the  deep  channel  relative  to  that  spread  out  in  a  thin 
layer  over  the  whole  bottom.  But  even  in  this  way  it  is  not 
always  possible  to  attain  an  automatic  flow,  as  the  inclination  is 
still  too  small,  and,  moreover,  on  account  of  its  fluidity,  the  sludge 
assumes  a  horizontal  surface  and  does  not  flow  from  the  sides 
into  the  channel  and  so  into  the  sump,  but  spreads,  rather,  in  a 
broad  stream  over  the  whole  of  the  bottom.  The  channel,  also, 
with  its  steep  sides  and  curved  invert,  renders  subsequent  clean- 
ing by  hand  more  difficult. 

To  facilitate  this,  or,  possibly,  to  install  an  arrangement  for 
removing  sludge  which  will  be  described  further  on,  the  tank 
should  be  built  with  a  basket-shaped  cross-section,  or  with 
straight  lines  and  a  steep  diagonal  slope,  but  without  sharp 
edges  or  corners. 

In  Frankfort-on-the-Main,  to  entirely  avoid  subsequent  clean- 


32 


SEWAGE  SLUDGE 


ing  by  hand,  two  sludge  sumps  were  constructed  in  a  tank 
135.8  ft.  (41.4  m.)  long,  and  the  bottom  had  a  longitudinal 
inclination  of  1:10  toward  these  (Fig.  6).  The  diagonal  slope 

at  the  ends  and  in  the  middle 
was  1:3,  and  near  the  sumps 
1:2  (Fig.   7).     These  have  a 
diameter  of  8.2  ft.   (2.5  m.). 
Their    bottoms     are     conical 
with  an  inclination  of  1 :1.     In 
addition,  all  surfaces  exposed 
to  the  sewage  are  lined  with 
glazed  brick  or,  where  this  is 
not     possible,    as     on    small 
ri      rounded  angles,  with  smoothly 
|      dressed    sandstone.      In   this 
|      way  a  perfectly  automatic  flow 
•g,      of  sludge  toward  the  sumps  is 
g      secured.       Moreover,    a    pipe 
„      for  water  under  pressure  with 
•ji      many    connections    has   been 
J      laid,    by  the  aid  of  which   a 
J      thorough     cleaning    may    be 
3      effected  by  jets,  for  the  glazed 
g      surfaces      gradually     become 
|       coated    with    a   sticky    layer 
0      which  increases  the  friction  of 

o 

£      the  flowing  sludge. 

Further  attempts  to  divide 
the  bottoms  of  tanks  into 
separate  hoppers  facilitate  the 
flow  of  the  sludge  and  aid 
particularly  in  its  removal 
during  the  operation  of  the 
plant  and  will  therefore  be 
treated  of  later. 

In  general  every  plant  should 
be  so  constructed  as  to  abso- 
lutely prevent  any  deposit  of  sludge  in  the  receiving  chambers, 
branching  channels  and  entrance  galleries  by  ensuring  an  ample 
velocity  to  the  stream.  We  should  endeavor  to  simplify  the 
process  by  separating  the  sludge  at  as  few  places  as  possible 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    33 

because,  from  the  lack  of  suitable  provisions,  the  removal  of 
sludge  from  these  parts  of  the  plant  can  seldom  be  accom- 
plished without  interfering  with  the  operation. 

The  next  step  is  to  effect  the  concentration  of  the  sludge  in  the 
direction  of  the  pump  pit  by  mechanical  means,  and  thus  lower 
the  cost. 


FIG.  7. — Development  of  cross-section  of  chamber  with  pump  pit. 


In  the  plant  at  Bremen  a  kind  of  wooden  sludge  car  of  simple 
construction  is  used  (Fig.  8).  This  is  about  14.8  ft.  (4.5  m.) 
wide  and  runs  by  means  of  flanged  iron  wheels  on  substantial 
plunks  which  project  about  4  in.  (10  cm.)  above  the  wooden 
flooring  with  which  the  shallow  tank  at  that  place  is  provided. 
These  shallow  tanks,  which  are  about  65.6  ft.  (20  m.)  wide,  are 
therefore  divided  into  4  longitudinal  strips  corresponding  to  the 


34 


SEWAGE  SLUDGE 


width  of  the  car.  The  car  has  an  adjustable  squeegee  on  the 
forward  side  provided  with  a  strip  of  rubber  and  is  drawn  by  a 
rope  from  the  effluent  end  to  the  sump  at  the  inlet.  The  wind- 
lass is  turned  by  the  engine  which  operates  the  dredge,  and  as 
the  rope  runs  over  a  guide-pulley  it  can  be  used  for  all  four  tanks. 
The  return  movement  is  accomplished  by  another  wire  rope 
which  is  drawn  by  a  movable  windlass  operated  by  hand.  This 
motion  could  have  been  effected  by  the  engine  already  mentioned 
if  the  rope  were  led  through  guide  pulleys  around  the  tank,  as 


Rubber  Strip 
FIG.  8. — Car  for  removing  sludge.     (Bremen.} 

customary  with  steam  plows  which  are  driven  by  one  engine. 
To  move  the  car  across  the  end  of  the  tank  and  for  lifting  across 
the  wall  separating  two  tanks,  four  wide  rollers  are  used  which 
can  be  inserted  or  removed  by  means  of  screws.  Although 
where  there  is  an  accumulation  of  sludge,  the  car  must  make 
several  trips,  yet  78  to  92  cu.  yds.  (60  to  70  cbm.)  of  sludge  can 
be  removed  from  a  large  [commonly  32,300  sq.  ft.  (3000  sq.  m.)] 
and  nearly  horizontal  tank,  by  two  men  in  one  day,  while  for- 
merly it  required  nine  men  for  perhaps  three  days  to  do  this. 
Moving  the  car  across  of  course  makes  it  necessary  for  the  men 
to  get  into  the  tank,  but  this  is  only  at  the  effluent  end  where 
there  is  little  sludge  and  occupies  but  little  time. 

This  has  been  avoided  at  Bolton  by  constructing  in  each  of  the 
shallow  tanks,  328  ft.  (100  m.)  long,  a  sludge-pushing  car  (Fig.  9) 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     35 

which,  on  account  of  the  narrow  width  of  the  tank,  serves  for 
the  whole  cross-section.  It  is  said  that  all  the  sludge  in  the  tank 
can  be  removed  in  15  minutes.  It  is  doubtful,  however,  if  the 
sludge  can  be  removed  by  this  apparatus  without  also  drawing 
off  the  upper  layer  of  sewage;  for  the  watery  sludge,  on  account 
of  the  slight  difference  between  its  specific  gravity  and  that  of 
the  sewage  in  a  full  tank,  would  probably  rise  in  front  of  the  car 
and  flow  over  it,  aided  by  the  current  induced  by  the  motion. 
This  cannot  happen  with  an  empty  tank  on  account  of  the  density 


FIG.  9. — Ashton-apparatus  for  removing  sludge  from  shallow  tanks.      (Bolton.) 

of  the  sludge  with  its  contained  water.  With  a  rounded  cross- 
section  of  the  tank  such  an  apparatus,  modeled  after  a  canal- 
cleaning  car,  could  be  used  and  could  be  driven  by  a  light  movable 
windlass,  preferably  run  by  electricity.  The  whole  construction 
could  be  made  much  simpler  and  lighter  by  having  the  rope 
attached  to  the  squeegee  at  several  points.  The  question  of  a 
gain  in  efficiency  depends  upon  a  uniform  cross-section  for  the 
entire  length  of  the  tank. 

b.  REMOVAL  OF  SLUDGE  DURING  OPERATION 
1.  CONSTRUCTION 

The  somewhat  costly  and  troublesome  process  of  removing 
sludge  during  a  suspension  of  the  flow  led  early  to  the  devising 


36  SEWAGE  SLUDGE 

of  ways  and  means  for  simplifying  and  cheapening  the  work  by 
continuous  removal.  Wells  were  first  considered  for  this  purpose, 
because  the  comparatively  small  sludge  tank,  especially  in 
chemical  precipitation,  which  was  then  in  general  use,  with  its 
large  volume  of  sludge,  required  frequent  cleaning;  while  its 
form  offered  the  fewest  difficulties  to  continuous  removal. 

Its  principal  advantage  is  in  avoiding  the  costly  removal  of  the 
turbid  sewage,  which  in  most  places  must  be  drawn  off  by  pumps 
from  tanks  as  well  as  from  wells;  and  especially  where  the  process 
necessitates  frequent  cleaning  this  is  an  important  consideration. 
Moreover,  the  entire  plant  can  be  in  use,  while  otherwise  to  avoid 
overloading  it  must  be  constructed  of  greater  size  in  order  to 
allow  for  those  parts  which  lie  idle  during  cleaning.  It  can  be  so 
designed,  moreover,  that  only  the  dried  sludge  is  exposed, 
provided  closed  pipes  are  used  and  the  drying  is  done  by  a 
mechanical  process  described  later  on,  so  that  the  demands  of 
hygiene  are  more  completely  met  and  foul  odors  are  almost 
eliminated.  But  even  if  sludge  is  dried  in  the  open  air  this 
method  offers  great  advantages,  especially  if  the  places  for 
drying  are  at  some  distance  from  the  treatment  plant. 

A  disadvantage  in  most  plants  cleaned  during  operation  lies 
in  the  fact  that  their  sludge  contains  a  greater  amount  of  water, 
not  less  than  95  per  cent. 

Where  it  can  be  utilized  in  this  wet  condition  without  further 
transportation  or  where  ample  areas  for  drying  with  favorable 
sub-soil  and  location  are  available,  this  matter  is  of  less  import- 
ance. 

In  those  plants,  on  the  other  hand,  where  the  drying  or  hand- 
ling is  done  by  machines  whose  size  would  have  to  be  increased  to 
correspond  to  the  greater  volume  of  sludge,  one  should  consider 
whether  the  increased  efficiency  will  pay  for  the  greater  outlay. 
As  this  manner  of  removing  sludge  always  requires  a  material 
that  will  flow,  it  must  often  take  place  before  it  has  fully  settled. 
It  is  therefore  especially  adapted  to  thorough  sedimentation, 
that  is,  where  the  clarified  sewage  is  discharged  to  the  stream 
without  subsequent  treatment  and  consequently  must  not  be  in  a 
putrescent  condition.  The  sludge  shows  an  easily  flowing  con- 
sistency when  it  contains  a  small  amount  of  grease,  although  it 
contains  but  little  water.  The  Kremer  apparatus  is  therefore 
particularly  well  adapted  to  the  removal  of  sludge  during  opera- 
tion, because  we  have  with  it  the  separation  of  the  fats  and  cellu- 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    37 

lose  which  are  found  in  the  partly  clarified  upper  stratum  of  the 
liquid,  while  in  the  lower  part  we  have  the  descending  sediment. 

It  goes  without  saying  that  one  can  never  predict  with  cer- 
tainty regarding  any  of  these  details  that  all  the  sludge  will  be 
removed,  and  hence  that  there  is  no  more  putrescible  matter 
present. 

Removal  of  sludge  during  operation  is  effected  either -by  the 
construction  of  the  plant  or  by  the  introduction  of  some  special 
mechanical  device.  Sometimes  both  of  these  means  are  em- 
ployed. 

A  favorable  concentration  of  the  sludge  at  the  bottom  should 
be  aimed  at  in  the  design.  This  end  is  most  frequently  attained 


Fia.  10. — Dortmund  tank. 


with  wells.  As  already  mentioned,  these  are  almost  universally 
arranged  for  the  removal  of  the  sludge  without  preliminary 
emptying.  With  their  comparatively  small  dimensions  it  is 
usually  easy  to  give  the  bottom  such  an  inclination  that  the 
sludge  will  flow  by  itself  toward  the  suction  pipe  of  the  sludge 
pump  at  the  center.  A  slope  of  2  : 1,  as  is  found  in  the  so-called 
Dortmund  tank  (Fig.  10)  and  which  is  also  used  in  England, 
suffices  for  all  cases.  A  slope  of  less  than  45  degrees,  as  in  the 
sludge  well  constructed  in  the  clarification  tank  at  Frankfort, 
will  permit  a  slippery  sludge  to  slide  off  if  submerged.  The 
angles  between  the  vertical  walls  and  the  conical  base  should 
receive  especial  attention,  as  experience  indicates  that  the  sedi- 


38  SEWAGE  SLUDGE 

ment  in  the  sludge  settles  here.  This  can  be  prevented  to  a 
certain  extent  by  rounding  these  corners. 

Naturally,  the  degree  of  roughness  of  the  bottom  helps  deter- 
mine the  slope.  In  large  plants  it  is  well  to  make  experiments 
with  the  sludge  which  comes  from  the  sewage  to  be  treated, 
unless  the  slopes  have  been  determined  by  reliable  experiments 
with  different  kinds  of  sludge  on  different  surfaces  from  which  it 
slips  off  by  its  own  weight. 

It  should  furthermore  be  noted  that  in  course  of  time  a  sticky 
coating  is  deposited  on  the  smooth  surfaces,  reducing  their 
efficiency  very  considerably — especially  in  the  case  of  smooth 
enameled  or  glazed  surfaces  and  those  of  glass — and  that  their 
cleaning  necessitates  a  cessation  of  operation. 

The  cone  formed  at  the  bottom  of  the  wells  corresponding  to 
the  natural  slope  of  the  earth  will  not  suffice  for  a  free  removal  of 
the  sludge. 

It  has  been  shown  by  experiments  of  Schoenf elder  at  Elberfeld 
that  a  steeper  slope  is  required  to  secure  an  automatic  sliding  of 
the  sludge  if  removed  under  water  than  if  the  supernatant  liquid 
is  first  drawn  off,  and  that  special  precautions  should  be  taken  in 
the  process.  Here  it  was  observed  that  the  sludge  was  deposited 
in  horizontal  layers  not  of  uniform  thickness,  parallel  to  the 
bottom.  When  the  sludge  was  drawn  off  at  the  deepest  point  a 
funnel  was  formed.  After  this  the  sludge  failed  to  slide,  although 
having  a  slope  of  1:3;  but  this  did  occur  immediately  after  draw- 
ing off  the  supernatant  sewage.  The  explanation  of  this  is  that 
the  difference  in  weight  between  the  saturated  sludge  and  the 
turbid  sewage  above  is  too  slight  to  overcome  the  friction  of  the 
surface  at  the  bottom  and  of  the  surface  in  contact  with  the 
turbid  sewage;  for  the  weight  of  the  sludge  is  reduced  by  that  of 
the  displaced  sewage,  while  in  the  case  of  empty  tanks  the 
weight  of  the  sludge  becomes  effective.  The  funnel  mentioned 
gradually  closed  in  again  under  water  so  that  in  a  half  hour  it 
was  always  smooth  and  horizontal. 

This  was  confirmed  by  experiments  at  Cologne.  Here  the 
sludge  was  to  be  pumped  from  under  water  out  of  sumps  having 
a  slope  of  1  : 1.  It  soon  appeared  that  after  a  few  minutes  only 
water  came  out,  which  found  its  way  through  the  compact  sludge 
near  the  suction  strainer  and  carried  with  it  only  a  few  frag- 
ments of  sludge  which  it  was  able  to  dislodge.  This,  which  is 
also  confirmed  by  practice  and  experiments  elsewhere,  proves 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    39 

that  the  removal  of  sludge  under  water  and  without  stirring  up 
the  deposited  material  is  only  practicable  before  the  sludge  is 
firmly  settled  in  place.  The  composition  of  the  sludge  is  of 
importance  in  this  connection  as  the  greasy  material  forms  a 
light  but  firm  mass,  while  sludge  from  septic  tanks  which  is  kept 
in  motion  by  frequent  partial  removal,  as  in  the  Emscher  tank, 
and,  by  the  gases  rising  from  it,  can  easily  be  removed  during 
operation. 

In  these  plants,  which,  as  is  known,  are  a  combination  of 
short  sedimentation  tanks,  with  septic  chambers  below,  the 
difference  in  quality  between  the  fresh  and  septic  sludge  is 
taken  into  account  by  giving  the  floor  of  the  upper  part  a  slope 
of  1  1/2:1,  and  in  the  most  recent  structures  this  is  covered  with 
glass  plates  laid  on  reinforced  concrete  supports  to  lessen  the 
friction.  Such  precautions  are  necessary  in  order  to  induce  the 
settled  sludge  to  slide  down  in  thin  layers  to  the  lower  chamber 
as  soon  as  possible. 

As  the  experiments  of  Grimm  (which  led  him  to  introduce 
sedimentation  plates  in  the  tanks)  have  shown,  this  is  promoted 
by  the  fact  that  colloidal  matter  has  a  marked  tendency  to  form 
a  gelatinous  coating  by  friction,  or  even  by  contact,  with  a  solid 
body.  This  is  then  set  in  motion  on  the  steep  surface  by  gravity, 
and  in  rolling  down  carries  with  it  the  particles  of  sludge  which 
are  in  the  way.  In  order  to  convey  the  septic  sludge,  which 
fills  the  lower  tank  in  a  great  mass,  to  the  sludge  pipe  a  slope 
of  1 : 2  is  sufficient,  to  which  may  be  added  a  flushing  pipe  to  be 
described  later. 

The  many  other  forms  of  wells  which  have  been  constructed 
in  view  of  the  particular  end  to  be  reached,  and  especially  for 
chemical  precipitation  in  all  its  different  phases,  more  particularly 
with  reference  to  the  introduction  and  distribution  of  the  sewage, 
are  subject  to  the  same  principles  regarding  the  removal  of 
sludge  as  the  Dortmund  tank,  given  as  an  example.  The  same 
is  true  of  the  short,  shallow  tanks,  having  the  base  constructed  as 
a  pyramid,  with  sides  sloping  at  45  degrees  or  more  in  order  to 
facilitate  the  removal  of  sludge.  The  advantage  of  this  form  is 
made  evident  by  the  simplicity  with  which  the  desired  purpose  is 
effected. 

The  attempt  has  also  been  made  in  various  ways  to  remove 
the  sludge  from  the  ordinary  long  shallow  tank  while  in  use. 

In  Thorn  the  bottom  of  a  tank  fully  65.6  ft.  (20  m.)  long  and 


40 


SEWAGE  SLUDGE 


perhaps  26.2  ft.  (8  m.)  wide,  having  sides  with  a  slope  of  less 
than  45  degrees  was  divided  for  this  purpose  into  4  parts  (Fig.  II)1 
by  constructing  saddle-shaped  division  walls,  from  the  lowest 
point  of  which  the  sludge  was  led  under  water  pressure  to  the 
sludge  channel  which  was  used  in  common  for  two  tanks. 

A  similar  solution  by  the  use  of  hopper-shaped  bottoms  has 
been  employed  by  Schoenf elder  at  Elberfeld  (Fig.  12),  but  their 


0- 

Well  for 

Clarifying 
with  Lime 


Plan. 


Longitudinal  Section. 


Transverse  Section. 

Fia.  11. — Sedimentation  tanks  at  Thorn. 

dimensions  have  been  made  quite  different  for  the  following 
reasons:  As  the  largest  amount  of  sludge  settles  in  the  first 
quarter  of  the  tank,  as  was  observed  in  the  Cologne  experiments, 
the  last  hopper-shaped  compartments,  if  the  tank  were  composed 
of  compartments  of  equal  size,  would  require  very  much  longer 
to  fill  with  the  fine  sludge  than  the  first  ones  in  which  the  coarser 
constituents  were  settled  out.  In  this  way  a  separation  of  the 
sludge  according  to  its  composition  was  effected.  This  is 
particularly  valuable  because  the  fine  sludge,  on  account  of  the 


1  Fig.  11  is  taken  from  Salomon's  "Die  Stadtische  Abawasserbeseitigung  in  Deutschland," 
1907. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     41 


light  weight  of  particles  of  fat,  contains  the  most  grease  and 
can  later  be  manipulated  so  as  to  separate  this  out,  while  the 
coarse  sludge,  on  account  of  the  small  amount  of  grease,  can  be 
drained  more  quickly  and  easily.  For  this  reason  the  sludge 
tank  at  Elberfeld  is  divided  for  the  purpose  of  receiving  these  two 
kinds  of  sludge,  without,  however,  any 
use  being  made  of  the  device  as  yet. 
Another  advantage  is  that  the  hoppers 
for  the  fine  sludge  can  have  less  slope, 
on  account  of  its  greater  fluidity,  at 
least  in  the  upper  portion,  besides  being 
of  smaller  dimensions.  Experiments 
with  a  model  demonstrated  that  less 
slope  was  required  for  the  concentration 
of  the  sludge  under  water,  while  a  steeper 
one  was  required  to  avoid  the  formation 
of  funnels  while  forcing  it  out.  There- 
fore steeper  hoppers  were  inserted  to 
ensure  a  removal  of  the  sludge. 

The  easiest  way  to  measure  the  height 
of  the  sludge  in  wells  and  tanks  is  by 
lowering  a  sheet  iron  plate  attached  in 
a  horizontal  position  to  a  measuring 
chain.  By  the  increased  resistance  to 
the  vertical  movement  of  such  a  plate 
in  sludge  in  comparison  with  water  one 
can  determine  with  sufficient  exactness 
the  height  of  the  sludge  by  reading  from 
the  chain. 

It  is  not  sufficient  to  concentrate  the 
sludge  at  one  or  more  points  under  the 
sewage,  but  it  must  also  be  delivered, 
and  herein  lies  a  particular  difficulty 
in  the  removal  of  sludge  during  oper- 
ation. 

The  delivery  of  the  sludge  can  be 
accomplished  1.  by  suction  with  pumps,  vacuum  receivers  or 
similar  apparatus  in  the  same  way  as  in  its  removal  during 
suspension  of  operation;  2.  by  drawing  it  off  by  the  aid  of 
hydrostatic  pressure,  either  toward  a  deeper  channel  or  sludge 
well  or,  under  pressure,  through  a  rising  main,  so  that  it  is  dis- 


42  SEWAGE  SLUDGE 

charged  but  a  little  below  the  level  of  the  surface  of  the  sewage 
in  the  well  or  tank. 

The  insertion  of  a  sludge  well  in  making  use  of  vacuum  appa- 
ratus is  of  advantage,  as  in  this  way  a  uniform  flow  of  the  sludge 
is  procured  and  fluctuations  of  the  flow  resulting  in  the  entrain- 
ment  of  larger  amounts  of  water,  as  may  readily  occur  by  the 
intermittent  operation  of  such  an  arrangement,  may  be  avoided. 
Moreover,  this  permits  observation  of  the  amount  of  water  con- 
tained in  the  sludge  delivered  from  plants  to  which  it  is  adapted; 
which  is  otherwise  only  possible  at  the  end  of  the  rising  main, 
and  as  this  is  often  at  some  distance  from  the  clarification  plant, 
it  is  impracticable. 

In  forcing  sludge  through  a  rising  main  there  should  be  a 
difference  in  elevation  of  2.6  to  3.3  ft.  (0.8  to  1.0  m.)  between  the 
surface  of  the  sewage  and  the  discharge  end  of  the  pipe  with 
ordinary  settled  sludge.  With  the  Emscher  tank  this  should  be 
increased  to  5.0  ft.  (1.5  m.).  Here  two  flushing  pipes  are  pro- 
vided for  water  under  pressure  of  which  one,  forming  a  ring,  is 
perhaps  at  the  elevation  of  the  connection  between  the  cylinder 
and  the  conical  base,  and,  with  its  orifices  directed  downward, 
is  intended  to  assist  the  sludge  in  sliding  down  the  gentle  slope  of 
the  base.  The  second  terminates  opposite  the  entrance  to  the 
sludge  pipe  in  a  loop  with  three  orifices  directed  toward  the 
center.  This  serves  as  a  supplementary  aid  and  to  start  the 
flow  in  case  large  masses  of  grit  should  collect  there.  As  a  failure 
of  such  plants  is  usually  through  refusal  of  the  sludge  which  has 
accumulated  in  the  pipes  during  a  cessation  of  operation  to  flow 
after  opening  the  valve,  it  is  advisable  to  provide  the  sludge  pipe 
with  a  branch  pipe  from  the  water  main,  as  has  been  done  with 
the  Emscher  tanks,  so  that  the  pipe  may  be  filled  with  water 
after  each  emptying  of  sludge  and  the  remaining  sludge  forced 
back  to  the  tank.  By  means  of  these  pipes  positive  action  can 
be  secured  under  difficult  conditions,  as  the  author  can  testify. 
The  use  of  the  two  last-mentioned  pipes  is  always  well  wrhere 
there  is  difficulty  in  forcing  out  the  sludge,  especially  in  large 
plants  where  there  is  almost  always  water  under  pressure  avail- 
able for  flushing  purposes.  The  sludge  pipe  should  always  be 
as  straight  as  possible,  avoiding  sharp  curves,  which  cause  a  loss 
of  pressure. 

In  conveying  sludge  under  pressure  where  the  operation  is  not 
continuous,  care  should  be  taken,  in  starting  its  movements,  to 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     43 

avoid  the  formation  of  the  funnels  already  mentioned.  For 
although  the  water  exerts  a  uniform  pressure  on  the  nearly 
horizontal  surface,  the  vertical  column  of  sewage  over  the  pipe 
entrance  will  be  set  in  motion  by  the  sudden  opening  of  the 
valves  and  will  then  settle  and  so  increase  the  height  of  the 
column  above.  This,  with  the  friction  of  the  masses  of  sludge 
on  the  bottom  and  sides,  helps  in  the  formation  of  a  funnel. 

This  can  be  effectively  prevented  by  a  device  called  a  sludge 
cylinder  (German  Patent)  of  the  Company  for  Sewage  Purifica- 
tion (Berlin-Schoeneberg),  which  can  be  attached  to  their 


FIG.   13. — Kremer  apparatus. 


Kremer  apparatus  (Charlottenburg)  (Fig.  13).  The  bottom  of 
the  tank  converges  to  a  hopper  having  an  inclination  of  less  than 
60  degrees  with  the  horizontal,  and  is  continued  as  a  cylinder 
2  it.  7  1/2  in.  (0.8  m.)  in  diameter,  in  which  the  sludge  is  collected. 
The  sludge  pipe  ends  at  the  bottom  of  this  cylinder,  and  through 
this  the  sludge  is  removed  by  hydrostatic  pressure.  The  forma- 
tion of  funnels  with  the  resulting  discharge  of  sewage  is  not 
possible  in  the  narrow  cylinder,  so  the  sludge,  which  contains 
comparatively  little  water  in  the  Kremer  apparatus  (80  to  85 
per  cent.)  is  removed  from  the  tank  without  change  in  its  favor- 
able composition. 


44 


SEWAGE  SLUDGE 


2.  MECHANICAL  CONTRIVANCES  FOR  REMOVING 
SLUDGE  DURING  OPERATION 

These  mechanical  devices  may  be  classified  as  follows: 

1.  Those   which,   by   stirring,   mix   the   required   amount   of 

water  with  sludge  that  is  not  adapted  to  continuous  removal, 

or  at  least  assist  in  initiating  its  movement. 


—To  -the  Air  Pump 
-19  -Hie  Pipe  fvr  Draining 


pur*  ••••«••  rvn  •  *m=»  =  •=• 

Sewage   Influerrf-- 


FIG.  14. — Stirring  device  and  skimmer  in  sedimentation  tower  in  the 
lignite  process. 

2.  Those   which   collect   the   sludge   at   certain   points,   from 
which  it  may  be  readily  conveyed. 

3.  Those    which,    without    materially    affecting    its    settling, 
draw  the  sludge  off  from  the  place  where  it  has  been  deposited. 

The  stirring  devices  of  the  first  category  act  in  opposition  to 
the  principle  laid  down  at  the  beginning  of  the  section,  that  the 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    45 

consistency  .of  sludge  should  not  be  detrimentally  altered  in 
removal,  and  are  therefore  to  be  avoided  as  much  as  possible. 
They  are  sometimes  installed  later  if  the  sludge  does  not  flow  of 
itself  on  account  of  too  flat  a  slope  in  the  bottom.  If  removal 
takes  place  during  suspension  of  operation,  the  height  of  the 
stirring  device  above  the  bottom  should  be  adjustable  from 
above,  so  as  to  be  always  in  contact  with  the  top  layer.  (Wells 
at  Mairich,  Neustadt  O.-S.).  The  chief  occasion  for  their  use  is 
in  the  towers  used  in  lignite  treatment,  as  stirrers  can  only  be 
used  in  wells  or  towers.  They  are  commonly  provided  with  a 
device  for  maintaining  a  light  contact  with  the  sludge  surface 
and  are  kept  continuously  in  slow  motion  (Fig.  14).  The  flush- 


(Skimmer '" 


.Sludge  Gate 


FIG.  15. — Skimming  apparatus  and  sludge  tank  in  the  Kremer  apparatus. 

ing  pipes  with  water  under  pressure,  already  mentioned,  as  well 
as  arrangements  for  securing  the  flow  of  sludge  by  compressed 
air,  should  be  included  here.  Stirring  the  sludge  is  said  to  pro- 
mote its  digestion  in  Emscher  tanks,  but  disturbs  uniform  set- 
tling in  sedimentation  tanks. 

Arrangements  for  concentrating  sludge  are  based  upon  the 
same  idea  as  the  apparatus  described  for  use  during  suspended 
operation,  but  the  construction  may  be  lighter  as  the  volumes  of 
sludge  to  be  disposed  of  on  account  of  more  frequent  removal 
are  smaller  and  do  not  offer  so  much  resistance.  In  an  experi- 
mental Kremer  apparatus  at  Charlottenburg  a  simple  surface 
scraping  apparatus  was  found  serviceable  in  a  square  tank  with 
a  flat  bottom.  The  apparatus  (Fig.  15)  consists  of  a  scraper  in 
the  shape  of  a  board  which  can  be  turned  on  its  upper  edge 


46 


SEWAGE  SLUDGE 


.-•Supply  Channel 


Entrance          g"Dicimt 


Discharge 


Effluen+>  Channel 


Well  for 
Sludge 
Valve 


FIG.  16.— Spiral  shaped  sludge  collector.      (Fidler  Patent.) 


Sludge 
Discharge 

—  '^--Rotary  Sludge 
"^^      Discharge  Pipe 

FIG.  17. — Sedimentation  tank  (Candy  system)  with  rotary  sludge 
discharge  pipe. 


TREATMENT  AND   UTILIZATION  OF  SLUDGE     47 

during  its  reverse  motion.  This  scraper  is  operated  by  two 
vortical  rods  which  are  attached  to  a  small  four-wheeled  car 
running  on  rails  placed  over  the  upper  edges  of  the  tank  and 
moved  by  hand. 

The  scraper  in  the  stirring  device  of  the  lignite  process  towers 
operates  in  the  same  way,  and  for  wells  having  a  very  steep  base 
a  scraper  composed  of  a  vertical  scraping  board  which  forces 
down  the  particles  of  sludge^  adhering  to  the  walls  may  be  found 
useful.  The  motion,  which  can  be  transmitted  by  gearing, 
should  be  very  slow  in  this,  as  in  the  following  apparatus,  to 
avoid  the  formation  of  detrimental  currents. 

The  patent  sludge  collector  of  Fidler  works  by  collecting  the 
sludge  by  a  rotary  motion  and  can  be  installed  in  wells  having  a 
flat  bottom.  It  is  also  used  at  Bolton  in  a  long  rectangular  tank, 
where  several  such  appliances  are  placed  side  by  side.  On 
account  of  the  dead  corners  some  supplementary  hand  labor 
is  required,  however.  As  shown  in  Fig.  16,  the  sludge  gatherer 
consists  of  a  spiral-shaped  iron  band  which  is  set  in  motion  by  a 
hand-operated  gear  and  which  sweeps  the  sludge  to  the  center, 
from  which  it  is  drawn  off  by  suction  or  forced  out,  as  illustrated 
in  the  cut.  Twelve  large  wells  on  this  system  have  recently  been 
installed  at  Bury. 

The  arrangement  in  the  third  category  consists  of  a  perforated 
pipe  laid  close  to  the  bottom  and  connected  with  the  sludge  pipe 
and  into  which  the  sludge  is  forced  by  the  pressure  of  the  water. 
A  rubber  squeegee  can  be  attached  to  this  pipe  which  scrapes  the 
sludge  from  the  bottom.  Fig.  17  shows  such  a  pipe  in  a  well  of 
the  Candy  system.  This  is  also  connected  with  a  wall  scraper. 
Motion  is  derived  from  a  gear-wheel  operated  by  hand.  The 
same  principle  applied  to  a  shallow  tank  is  shown  in  Fig.  18, 
representing  a  plant  in  Hey  wood.  The  perforated  pipe  is  guided 
here  by  two  rack  rails  into  which  the  pinions  conveying  the 
motion  engage.  The  sludge  is  here  raised  up  by  a  siphon  over 
the  side  wall  and  into  a  channel  which  is  common  to  two  tanks. 
A  pressure  of  3  ft.  (0.90  m.)  is  sufficient.  The  siphon  is  started 
by  cutting  out  the  upper  part  by  the  valves  a  and  b  which  is  then 
charged  with  a  pump  operated  from  the  same  platform  as  the 
moving  machinery.  The  horizontal  suction  pipe  is  provided 
with  a  rubber  squeegee.  A  disadvantage  in  this  form  is  that 
important  moving  parts  of  the  machine  are  submerged. 

These  contrivances  for  removing  sludge  require  for  reliable 


48 


SEWAGE  SLUDGE 


operation  a  liquid,  easily  flowing  sludge  and  must  therefore  be 
cleaned  out  every  day,  or  at  least  every  two  days.  The  sludge 
obtained  contains  about  97  per  cent,  of  moisture,  as  the  turbid 
sewage  finds  its  way  to  the  entrance  more  readily  than  the  sludge 
is  drawn  off  from  the  bottom.  With  a  pipe  which  rotates  about 
a  central  axis  it  follows  that  the  motion  is  slower  near  the  center 
of  the  tank,  while  the  friction  is  reduced  on  account  of  the  short 
length  of  the  pipe;  therefore  more  sewage  is  taken  in  here.  As 
this  sludge,  with  97  per  cent,  of  moisture,  has  twice  the  volume 
of  that  with  94  to  95  per  cent,  in  the  Dortmund  tank  when  in 
operation,  with  the  same  amount  of  dried  matter,  the  use  of  this 
method  of  collecting  sludge  is  seldom  to  be  recommended. 
Besides,  with  a  larger  proportion  of  mineral  matter,  as  often 


FIG.  18. — Movable  sludge  discharge  pipe  for  sedimentation  tanks. 

occurs  after  a  thunder  storm,  the  operation  is  more  difficult  and 
uncertain.  In  the  Fidler  system  this  is  not  the  case.  As  sludge 
containing  less  water  is  secured  here,  while  its  delivery  remains 
the  same,  this  should  have  the  decided  preference. 

While  the  aim  of  all  these  structural  or  mechanical  arrange- 
ments for  removing  sludge  during  operation  is  to  collect  the 
material  in  clarification  tanks,  or  at  least  to  draw  it  from  these 
directly,  there  are  some  which  enclose  it  in  a  special  chamber  or 
compartment  without  removal  of  the  supernatant  sewage,  the 
intention  being  to  prevent  subsequent  admission  of  the  turbid 
sewage  and  its  mixture  with  the  sludge  and  to  secure  the  latter 
as  free  as  possible  from  moisture. 

It  is  well  to  consider  here  the  introduction  of  a  partition  wall 
with  a  valve  (sluice  gate)  leading  into  the  sludge  chamber  which 
has  been  found  valuable  in  the  Kremer  apparatus  at  Charlotten- 
burg  (Fig.  15)  and  has  also  been  used  in  similar  plants  by  the 
Sewage  Purification  Company.  This  permits  a  discharge  of  the 
accumulated  sludge  with  no  fear  that  the  turbid  sewage  will  pass 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    49 

out  with  it.  The  sludge  can  be  drawn  off  from  here,  in  which 
case  a  pipe  for  the  admission  of  air  should  be  inserted  from  the 
sludge  tank  to  above  the  surface  of  the  sewage,  or  it  may  be 
forced  out  by  the  introduction  of  compressed  air. 

Grimm  (Gesundheits  Ingenieur,  1909)  attempts  to  apply  the 
foregoing  principle  to  shallow  ;tanks  in  a  somewhat  different 
way.  The  bottom  is  divided  into  hoppers  10.76  sq.  ft.  (1  sq.  m.) 
in  area  with  side  slopes  of  45  degrees,  from  whose  lower  points 
vertical  pipes  lead,  each  row  of  which  is  connected  by  a  trans- 
verse pipe  (Fig.  19).  The  sludge  slides  into  these  pipes,  which 
have  a  diameter  of  3.9  to  5.9  in.  (10  to  15  cm.)  and  may  increase 
in  size  at  the  bottom,  and  its  separation  is  facilitated  by  hoods  or 
plates,  according  to  Travis'  theory,  through  which  the  water  is 


Fio.  19. 

forced  out.  The  height  of  the  accumulated  sludge  can  be 
observed  for  each  transverse  row  of  hoppers  in  a  glass  stand  pipe 
connected  with  the  sludge  pipe  at  the  traverseable  sludge  pas- 
sage-way.1 When  the  depth  is  sufficient,  the  tops  of  these 
sludge  pipes  are  closed  by  plugs  carrying  an  air  pipe  reaching 
above  the  surface  of  the  sewage  and  rendered  accessible  by  a 
movable  foot  bridge.  When  the  valve  of  the  sludge  pipe  at 
the  passage-way  is  opened,  the  sludge  is  discharged  by  gravity 
or  suction,  while  air  enters  by  means  of  the  aforesaid  air  pipes 
through  the  plug  valves.  When  the  sludge  is  drawn  out  and  the 
gate  valve  closed  the  pipes  will  fill  again  with  turbid  sewage  by 
opening  the  plug  valves  and  are  then  ready  to  receive  sludge 
once  more.  By  this  arrangement,  if  the  plug  valves  are  tight 
enough,  the  subsequent  entrance  of  turbid  sewage  is  prevented; 
that  is,  a  dry  sludge  is  secured  by  allowing  it  to  accumulate  in 
a  thick  layer.  By  providing  a  steep  slope  in  the  cross  pipes  the 

1  In  the  division  wall  between  the  tanks.     Trans. 
4 


50  SEWAGE  SLUDGE 

flow  will  be  facilitated.     In   order  to   realize   all   advantages, 
however,  this  plant  requires  conscientious  superintendence. 

A  company  for  the  purification  of  water  and  sewage  at  Neu- 
stadt  a.  d.  H.  has  a  patented  device  called  a  sewage  preparer 
which  prevents  the  sludge,  that  has  accumulated  in  the  sludge 
channel  of  a  short  tank  having  steep  slopes  on  each  side,  from 
passing  through  the  tank  with  the  current,  by  a  series  of  hori- 
zontal shutters  operated  from  above.  This  is  said  to  reduce 
the  friction  in  continuous  treatment  and  facilitate  sliding.  If 
the  shutters  are  laid  flat  they  shut  off  the  sludge  channel  on  the 
side  of  the  tank,  leaving  an  entrance  only  at  the  Upper  end.  By 
opening  the  gate  valve  sludge  will  be  forced  out  of  the  channel 
by  the  pressure  of  the  sewage  above,  as  though  from  a  tube. 
It  is  doubtful,  however,  whether  the  shutters  in  the  lower  part 
of  the  sludge-filled  tank  can  be  closed  tightly  enough  to  prevent 
the  turbid  sewage  from  mixing  with  the  sludge  in  large  quan- 
tities, especially  in  the  neighborhood  of  the  outlet,  as  the  moving 
parts  are  mainly  under  water  and  cannot  be  constantly  watched. 

c.  CONTRIVANCES  AND  CONDUITS  FOR  CONVEYING  SLUDGE 

Various  contrivances  have  been  employed  to  remove  the  grit 
from  clarification  tanks  in  case  the  sludge  cannot  be  forced  out 
by  the  pressure  of  the  sewage  above.  These  are: 

1.  Dredges. 

2.  Pumps. 

3.  Vacuum  apparatus. 

4.  Various  other  contrivances. 

1.  As  already  remarked,  dredges  are  chiefly  used  to  clean  out 
grit  chambers.     The  transport  of  the  material  in  large  plants 
is  often  accomplished,  by  belt  conveyors. 

Dredges  should  not  be  used  to  remove  sludge  from  clarification 
tanks,  especially  during  operation,  as  a  thorough  cleaning  with 
them  is  not  possible.  It  is  also  better  to  use  other  contrivances 
in  sludge  wells,  on  account  of  the  dirtiness  of  the  operation. 

2.  In  using  piston  pumps  the  great  difficulty  encountered  is  to 
keep  them  water-tight,  as  they  soon  become  worn  out  by  the 
sand  brought  in.     The  valves,  also,  often  interrupt  operation 
and  should  therefore  be  placed  where  they  are  readily  accessible. 
Hence  it  is  an  advantage  to  have  a  good  grit  chamber  for  the 
protection  of  the  pumps.     It  is  also  to  be  noticed  that  in  very 


TREATMENT  AND   UTILIZATION  OF  SLUDGE     51 

sandy  sludge  the  suction  lift  is  reduced,  so  that  pumps  and 
vacuum  apparatus  must  be  placed  lower.  Coarse  bar  screens 
are  also  necessary  with  all  kinds  of  pumps,  to  prevent  pieces  of 
wood  or  other  coarse  material  from  injuring  the  pumps.  If  these 
coarse  screens  are  placed  in  the  tank  in  front  of  the  sludge 
channel  or  sump,  as  at  Mannheim  or  Munich-Gladbach,  the 
amount  of  the  screenings  will  be  less,  because  much  of  it,  especi- 
ally lumps  of  fecal  matter,  disintegrate  in  the  tank;  but  their 
removal  from  the  bottom  is  less  cleanly  and  more  troublesome. 
It  is  advisable  to  install  a  sludge  well  where  the  sludge  is  not 
removed  during  continuous  treatment,  as  the  pumping  plant 
may  then  be  made  smaller  without  increasing  the  time  of  re- 
moval. With  a  very  viscous  sludge  it  is  of  advantage  to  have  a 
stirring  device  operated  in  connection  with  the  pump. 

Diaphragm  pumps,  such  as  those  furnished  by  Bopp  and 
Reuther  of  Mannheim  for  the  clarification  plant  at  Hanover,  are 
superior  to  ordinary  piston  pumps  and  are  also  much  used  else- 
where. Here  the  piston  works  in  clean  water,  which  conveys 
the  pressure  through  diaphragms  to  the  sludge  which  is  to  be 
delivered. 

Centrifugal  pumps  are  also  well  adapted  to  the  transport  of 
sludge,  but  it  is  important  that  the  impeller  should  be  accessible. 
With  sandy  sludge  there  is  a  great  amount  of  wear  on  the  packing 
rings.  They  are  particularly  serviceable  in  pumping  roily  water 
because  of  their  simple  design,  especially  when  operated  by  elec- 
tricity. By  means  of  connecting  pipes  they  can  also  be  used  for 
reserve  power  in  pumping  sludge. 

For  small  plants  and  as  a  reserve  in  those  where  sludge  is 
propelled  by  hydrostatic  pressure,  the  well-known  diaphragm 
pumps  are  very  useful. 

3.  With  a  vacuum  apparatus  it  is  well  to  connect  two  receivers, 
so  that  the  air  drawn  from  one  will  serve  to  force  the  sludge  from 
the  other,  which  was  previously  filled.  The  air  pump  must 
therefore  be  arranged  to  act  as  a  vacuum  and  force  pump. 
The  operation  of  the  valves  is  then  automatic.  To  prevent  the 
sludge  from  running  into  the  air  pump  a  U-shaped  pipe  should 
be  inserted  between  it  and  the  receiver,  the  top  of  which  should 
be  at  least  33  ft.  (10  m.)  above  the  highest  level  of  the  sludge. 
Vacuum  receivers  are  especially  necessary  where,  from  absence 
of  grit  chambers  and  screens,  much  coarse  material  is  mixed  with 
the  sludge.  They  may  also  be  used  to  propel  the  sludge  for 


52  SEWAGE  SLUDGE 

long  distances.  In  Oppeln  the  sludge  is  forced  3600  ft.  (1100  m.) 
in  a  pipe  12  in.  (300  mm.)  in  diameter.  They,  however,  occupy 
more  space  than  pumps. 

The  patent  Wegner  vacuum  wagon  works  on  the  same  principle 
as  the  vacuum  receiver.  Here,  in  a  portable  receiver  which 
can  be  used  to  transport  the  sludge,  the  air  is  rarified  by  ex- 
ploding benzine,  causing  the  sludge  to  be  sucked  in.  This 
apparatus  is  especially  recommended  for  use  with  very  small 
plants  where,  because  of  the  proximity  to  improved  property 
or  public  works,  it  is  desirable  to  have  an  odorless  removal  of 
the  sludge  without  the  necessity  of  constructing  a  special  plant 
for  pumping  it.  As  the  receiver  can  be  brought  to  any  tank  or 
well,  the  cost  of  sludge  pipes  is  saved.  The  removal  is  also  odor- 
less. This  mode  of  operation  has  been  found  very  satisfactory 
at  Merseburg.  In  some  cases  the  pneumatic  apparatus  used  for 
cesspool  cleaning  can  be  used  in  the  same  way. 

4.  The  steam  ejector,  among  other  contrivances,  should  be 
mentioned  next.  This  has  not  been  found  useful  in  cleaning 
grit  chambers  on  account  of  the  consistency  of  the  detritus. 
This  difficulty  might  not  hold  in  the  case  of  the  liquid  sludge 
from  tanks,  but  the  disadvantage  of  the  large  consumption  of 
steam  outweighs  the  advantage  due  to  its  simple  construction 
and  the  small  amount  of  room  taken  up.  In  sewerage  systems 
in  which  the  sewage  is  lifted  by  compressed  air  their  use  may  be 
considered. 

In  the  recently  installed  plant  at  Siegen  an  effort  was  made  to 
convey  the  detritus  of  the  grit  chamber  which  lies  at  a  high 
elevation,  by  means  of  a  horizontal  spiral  screw  delivering  into 
a  tip  car  at  the  outlet  end,  without  interrupting  operation  of  the 
plant,  but  as  yet  without  success. 

Sludge  pipes  should  always  be  accessible  on  account  of  the 
liability  to  stoppage,  and  should,  therefore,  not  be  enclosed  in 
masonry  for  any  great  length.  It  should,  moreover,  be  possible 
to  produce  a  strong  scouring  current  by  water  pressure  or  by  the 
aid  of  sludge-propelling  devices.  Sometimes  an  open  traversable 
channel  between  the  tanks,  in  which  the  pipes  are  laid  (Elberfeld) 
or  into  which  the  short  sludge  pipes  open  from  the  side  (Mairich 
plant  at  Guben  and  Ohrdruf),  serves  as  a  sludge  channel.  This 
has  the  advantage,  where  sludge  is  removed  during  operation, 
that  one  can  see  the  amount  of  water  removed  as  soon  as  it 
comes  out  and  act  accordingly.  The  advantage  of  the  possibility 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     53 

of  using  a  pipe  which  opens  into  the  sludge-well  as  a  siphon  to 
produce  suction  is,  however,  lost. 

The  open  channels  to  convey  the  sludge  to  the  drying  beds, 
which  are  intended  to  be  used  in  the  day  time,  should  have  the 
maximum  hydraulic  radius  in  order  to  reduce  the  friction. 

The  inclination  which  should  be  given  pipes  and  channels  to 
secure  a  flow  of  sludge  without  assistance  depends  upon  its 
nature  and  should  not  be  too  light.  Very  liquid  sludge,  with 
about  95  per  cent,  of  water  and  but  little  sand,  may  under  some 
circumstances  be  given  a  slope  of  1:100,  but  1:80  is  better. 
For  sludge  obtained  with  interrupted  operation  a  fall  of  1:40 
to  1:50  is  necessary.  The  plants  of  the  Emscher  Association 
have  grades  of  1 : 20  to  1 : 40,  while  the  pipe  conduits  at  Elber- 
f eld  have  1:30. 

Enclosed  pipes  are  always  preferable  to  open  channels  for 
hygienic  reasons,  especially  for  long  distances.  Moreover,  the 
sludge  is  more  readily  moved  and  deposits  of  sludge  can  be  more 
easily  flushed  out. 

The  valves  in  the  sludge  pipes  should  be  strong  and  simple. 
Those  which  have  a  bearing  on  one  side  only  should  have  the 
operating  screw  on  the  outer  side  engaging  in  a  rack  on  one  side 
only.  Valves  in  pipes  should  be  designed  without  a  bottom 
groove,  and  should  bear  on  the  narrow  edges  of  the  disc  in  order, 
to  prevent  an  accumulation  of  coarse  material  which  would 
prevent  the  valve  from  closing. 


CHAPTER  IV 
REDUCTION  OF  THE  WATER  IN  SLUDGE 

The  large  amount  of  water  in  sludge  is  a  drawback  to  its  use  in 
any  way  and  reduces  its  value  on  account  of  the  work  involved  in 
its  reduction.  If  it  is  used  while  wet  the  valueless  water  it  con- 
tains limits  the  area  to  which  it  can  be  applied,  on  account  of  the 
increased  cost  of  transportation.  Moreover,  this  liquid  condi- 
tion adds  to  the  difficulty  of  transportation,  as  this  can  only 
be  accomplished  in  water-tight  vessels,  and  temporary  storage 
in  the  field  is  impracticable  without  much  preparation  and 
apparatus. 

With  perhaps  75  per  cent,  of  moisture  it  can  be  loaded  with  a 
shovel  and  does  not  require  a  perfectly  water-tight  vessel. 
With  60  per  cent,  of  moisture  it  is  quite  firm  and  resembles  damp 
garden  mould.  It  should  therefore  be  reduced  to  this  condition. 

Fig.  20  shows  at  a  glance  the  relation  between  the  amount  of 
water  removed  and  the  consequent  reduction  of  volume.  The 
curves  represent  sludge  of  different  degrees  of  original  moisture, 
volumes  in  per  cent,  of  that  from  which  the  water  has  not  been 
extracted  being  represented  by  ordinates,  while  the  abscissas 
represent  the  degree  of  de-watering  in  percentage  of  moisture 
contained  in  the  whole.  The  horizontal  line  limits  the  volume 
of  the  dried  residue,  which  remains  constant.  The  distance  of 
the  curve  from  this  line  gives  the  amount  of  water  contained  in 
the  sludge. 

It  can  be  seen  here  how  much  water  should  be  removed  to 
reduce  the  amount  in  a  wet  sludge  by  10  per  cent,  and  how  the 
quantity  of  water  necessary  to  reduce  the  moisture  to  a  given 
percentage  rapidly  diminishes.  For  example,  from  100  Ibs.  of 
sludge  containing  90  per  cent,  of  water  (Fig.  20)  50  Ibs.  of  water 
must  be  removed  to  reduce  it  to  80  per  cent.,  while  reducing  the 
same  original  volume  from  60  to  50  per  cent,  requires  the  removal 
of  but  5  Ibs.,  and  from  30  to  20  per  cent,  only  1.8  Ibs. 

A  comparison  of  the  different  curves  shows  by  the  steep  slope 
on  the  right  hand  side — that  is,  at  the  commencement  of  de- 

54 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     55 

watering — how  important  it  is  for  subsequent  drying  and  treat- 
ment in  general  to  have  the  sludge  as  dry  as  possible  from  the 
beginning;  for  the  increase  in  the  water  content  of  the  sludge 
corresponding  to  an  increase  of,  say,  5  per  cent,  of  moisture, 
has  a  varied  effect,  according,  as  we  obtain  a  sludge  containing 
80  per  cent,  instead  of  75  per  cent,  or  sludge  of  95  per  cent, 
instead  of  90  per  cent,  moisture.  This  is  also  of  importance  in 
considering  the  arrangements  for  treating  the  sludge  described 
in  the  last  section. 


10     10     30    40    50    60    70     80    90 
Amount  of  Water  in  the  Mass  afrer  -the  Watering, 

{95% 
90%  — 
80%  — 
60% 

FIG.  20. — Reduction  of  volume  in  sludge  with  extraction  of  water. 

On  the  other  hand,  it  is  recognized  that  drying  beyond  50  per 
cent.,  at  least  with  the  very  wet  sludge  from  tanks,  has  but  a 
slight  effect  on  the  reduction  of  volume,  and  for  this  reason  alone 
further  de-watering  is  not  warranted. 

For  example,  sludge  originally  containing  95  per  cent,  of  water 
has,  when  dried  to  60  per  cent.,  but  1/8  of  the  original  volume, 
and  so  the  cost  of  transportation  is  correspondingly  lessened, 
and  the  extent  of  its  use  as  a  fertilizer  is  increased,  as  the  amount 
of  dried  material,  which  alone  is  of  value,  remains  unaltered. 

The  following  requirements  should  be  observed  in  the  process 
of  de- watering: 

1.  The  drying  should  not  involve  too  great  an  expense,  so  that 
the  expected  increase  in  value  of  the  product  is  not  lessened. 
This  may  be  accomplished  by  removing  the  water,  by  incinera- 


56  SEWAGE  SLUDGE 

tion,  for  instance,  thus  facilitating  its  transportation  and  a  more 
rational  utilization. 

2.  It  should  be  effected  rapidly,  especially  when  done  at  the 
plant  or  in  the  neighborhood  of  habitations,  in  order  to  avoid 
accumulations  of  filth. 

3.  The  operation  should  produce  no  nuisance  in  the  neighbor- 
hood from  foul  odors  or  otherwise. 

4.  Handling  of  the  sludge  by  workmen  should  be  avoided,  for 
reasons  already  stated. 

The  methods  of  removing  the  water  are  as  follows: 

a.  Drying  in  the  air. 

b.  Drying  by  filter  presser. 

c.  Drying  by  centrifugal  machines. 

d.  Other  methods  of  reducing  the  moisture. 

a.  DRYING  IN  THE  AIR 

Drying  in  the  air  is  the  process  most  commonly  employed, 
especially  with  small  plants.  For  this  purpose  the  sludge  is 
conducted  into  shallow  basins.  These  are  surrounded  by  earthen 
embankments  or,  less  frequently,  by  slope  paving,  wooden  sides 
or  solid  walls.  The  drying  is  effected  in  part  by  evaporation  of 
the  water  and  partly  by  its  drainage  into  the  underlying  soil. 

If  this  is  porous,  subdrainage  at  a  depth  of  about  2.3  ft.  (0.7 
m.)  with  a  small  distance  between  the  separate  lines  of  pipe  is 
sufficient.  Sometimes  even  this  is  unnecessary.  In  some  cases, 
however,  an  artificial  construction  of  the  bottom,  similar  to  a 
filter,  is  necessary,  as  with  the  natural  subsoil  the  accumulation  of 
sludge  increases,  so  that  it  becomes  necessary  to  remove  and 
renew  it.  The  depth  of  such  a  filter  is  usually  15  to  24  in.  (40  to 
60  cm.).  The  drainage  channels,  about  4  to  6  in.  (10  to  15  cm.) 
wide,  are  laid  with  open  joints  at  a  distance  of  from  4  to  10  ft. 
(1.2  to  3.0  m.)  apart  and  are  covered  to  a  depth  of  12  to  16  in. 
(30  to  40  cm.)  with  cinders  from  boilers,  pebbles  or  coarse  gravel. 
A  thick  covering  of  fine  cinders  or  screened  gravel  [2  to  4  in. 
(5  to  10  mm.)  in  size]  follows  this,  4  to  6  in.  (10  to  15  cm.)  in 
depth.  This  layer  is  to  prevent  the  sludge  from  penetrating 
further  into  the  filter.  As  the  topmost  layer  becomes  choked 
with  sludge  and  portions  of  it  are  carried  off  with  the  dried  sludge, 
it  has  to  be  renewed  from  time  to  time.  To  prevent  this  the  bed 
may  be  covered  with  heavy  stone  paving,  or,  as  at  Leipzig,  with 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     57 

a  layer  of  bricks.  The  joints  are  then  merely  filled  with  sand. 
As  all  the  water  drains  through  these  comparatively  narrow 
spaces  they  soon  become  clogged  with  sludge,  and  the  entire 
pavement  must  be  taken  up  and  renewed. 

The  liquid  drained  off,  which  is  usually  somewhat  turbid, 
owing  to  particles  of  sludge,  and  also  putrescible,  may  be  led  to 
the  intake  of  the  clarification  plant  and  treated  again.  As  the 
volume  is  comparatively  small,  it  does  not  alter  to  any  great 
extent  the  sewage  to  be  treated.  In  many  cases  this  is  impos- 
sible without  long  conduits  or  even  devices  for  lifting  it,  espe- 
cially where  the  sludge  is  removed  by  hydrostatic  pressure  and 
brought  by  gravity  in  open  conduits  to  the  drying  bed.  In  this 
case  a  small  supplementary  tank  for  subsequent  sedimentation 
or  a  filter  (Elberfeld)  for  the  sludge  liquor  is  advisable.  If  the 
sewage  is  subjected  to  subsequent  purification  by  contact  beds 
or  sprinkling  filters  the  sludge  liquor  can  receive  further  treat- 
ment there. 

The  liquor  from  drying  beds  after  septic  treatment  requires 
no  further  treatment  and  may  be  led  directly  to  the  outfall,  being 
odorless,  clear  and  nonputrescible,  as  shown  by  the  plants  of  the 
Emscher  Association.  By  this  method  most  of  the  water  sepa- 
rates out  in  the  first  12  hours  and  the  sludge  floats  on  account  of 
the  expansion  of  the  contained  gases,  while  a  layer  of  clear  water 
is  formed  underneath.  The  water  should  be  drained  through 
the  filter  as  quickly  as  possible,  for  after  the  gas  has  been  given 
off,  at  the  end  of  15  or  20  hours,  the  sludge  sinks,  due  to  its 
specific  gravity,  and  the  water  rises  over  it.  The  same  phenom- 
enon is  observed  in  the  deep  sludge  pit  at  Leipzig,  except  that 
here  the  water  is  not  drained  off  from  below,  but  is  allowed  to 
evaporate  after  it  has  risen  above  the  sludge. 

With  very  greasy  sludge  it  is  sometimes  impossible  to  draw 
off  the  water  at  the  bottom,  as  at  Frankfort-on-the-Main  and 
Mannheim,  as  the  particles  of  settled  sludge  form  an  impenetrable 
layer.  The  small  amount  of  water  on  top  can  then  only  be 
drained  off  in  as  many  places  as  possible  through  openings  in  the 
surrounding  walls,  which  can  be  closed. 

If  fresh  sludge  is  discharged  on  top  of  beds  of  partly  dried 
sludge,  as  can  scarcely  be  avoided  where  the  sludge  is  seldom 
cleaned  out,  it  will  dry  more  slowly  on  account  of  the  heavier 
liquid  beneath.  The  boundaries  of  the  basins  can  be  wholly  or 
partially  made  of  a  sort  of  woven  brush-work,  thus  obtaining  a 


58  SEWAGE  SLUDGE 

lateral  removal  of  the  water  through  these,  boundaries  (Elber- 
feld,  Halberstadt) .  Nevertheless,  the  efficacy  of  this  mode  of 
draining  the  sludge  is  greatly  diminished  by  the  fact  that  the 
rapid  delivery  of  the  water  through  the  brush  and  the  influence 
of  the  air  in  drying  the  accumulation  of  deposited  matter  at  the 
sides  soon  make  a  nearly  impervious  layer,  because  the  moisture 
does  not  come  rapidly  enough  from  the  interior.  At  Halber- 
stadt, therefore,  it  is  not  considered  advisable  to  retain  this 
device. 

At  Bremen  the  basins  were  subdivided  for  this  purpose  by 
perforated  planks,  between  which  narrow  passages  were  left  for 
drawing  off  the  liquid  that  leaked  through  and  for  removing  the 


FIG.  21. — Sludge  beds  of  the  Recklinghausen  Clarification  Plant. 

dried  sludge;  but  this  device  was  removed  because  the  sludge 
found  its  way  through  the  holes,  making  its  handling  mo're 
uncleanly,  without  resulting  in  a  more  rapid  drying. 

If  this  is  to  be  accomplished  the  sludge  should  be  brought 
into  the  basin  in  thin  layers,  6  to  10  in.  (15  to  25  cm.)  in  thickness, 
and  the  basin  should  be  filled  as  rapidly  as  possible,  in  order 
that  the  free  removal  of  the  water  at  the  bottom  may  not  be 
made  more  difficult  by  sludge  which  has  had  its  moisture  drained 
off.  Small,  shallow  tanks  are  therefore  to  be  preferred.  These 
can  be  economically  provided  in  small  installations  by  construct- 
ing a  border  of  planks  placed  on  edge.  In  this  way  the  excess 
area  required  by  earthen  embankments  is  utilized.  (See  Fig.  21.) 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     59 

In  the  cut,  which  shows  the  plant  of  the  Emscher  Association  at 
Recklinghausen,  the  sludge  running  into  an  empty  compartment 
may  be  plainly  seen  on  the  right.  The  sludge  is  forced  out  by 
hydrostatic  pressure  from  6  Emscher  tanks  lying  beyond.  In 
order  to  maintain  a  uniform  depth  of  'the  layers  the  bottom 
should  be  made  horizontal,  as  the  sludge,  due  to  its  fluid  nature, 
assumes  a  horizontal  position. 

A  disadvantage  of  plants  with  small,  shallow  basins  lies  in  the 
greater  cost  of  removing  the  dried  sludge,  as  this  comes  in  thin 
layers  and  necessitates  frequent  re-location  of  the  rails  for 
transportation,  unless  these  are  laid  on  an  elevated  trestle,  as  at 
Recklinghausen.  This  is  obviated  by  the  use  of  wheelbarrows. 

In  selecting  a  location  for  the  drying  beds  care  should  be  taken 
that,  with  a  porous  soil,  there  are  no  wells  in  the  vicinity  that 
can  be  contaminated  by  infiltration. 

Especial  care  should  also  be  taken  with  reference  to  odors  and 
the  plague  of  flies.  These  nuisances  have  again  and  again  led  to 
attempts  to  replace  the  cheap  method  of  drying  in  the  air  by 
others,  even  though  more  expensive. 

As  partial  decomposition  accompanies  the  operation  of  drying 
the  foul  odors  caused  by  gases  arising  from  the  sludge,  especially 
in  the  summer,  give  much  discomfort  to  persons  working  or 
living  near  the  plant.  It  may  even  result  in  lowering  the  value 
of  land  in  the  vicinity  and  cause  great  expense  for  indemnifica- 
tion. Likewise  the  plague  of  flies  is  very  troublesome  in  the 
neighborhood,  as  the  fermenting  sludge  offers  an  admirable 
breeding  place  for  all  kinds  of  flies  and  gnats. 

An  attempt  has  been  made  to  prevent  this  nuisance  by  adding 
some  substance  to  the  sludge.  Such  substances  are  either  in- 
tended to  stop  putrefaction  or  else  to  form  a  cover  to  the  sludge. 

In  Cassel,  for  example,  as  well  as  in  other  places,  lime  has  been 
successfully  added  to  the  sludge  in  the  basins  and  at  the  outlet 
in  the  proportion  of  6.8  Ibs.  per  cubic  yard  (4  kg.  per  cbm.).  The 
nuisance  of  flies  is  done  away  with  in  this  way,  but,  at  the  same 
time,  the  fertilizing  quality  of  the  sludge  is  reduced.  In  Frank- 
fort-on-the-Main,  however,  the  addition  of  quick  lime  and  chloride 
of  lime  has  not  had  the  desired  result. 

Peat  is  found  particularly  desirable  as  a  covering,  and  also  to 
prevent  odors,  besides  aiding  the  process  of  drying  by  absorption 
of  the  water.  It  is  used  in  many  places,  especially  where  it  is 
cheap.  In  order  to  prevent  putrefaction  it  should  be  intimately 


60  SEWAGE  SLUDGE 

mixed  with  the  sludge  in  large  quantities.  This  is  not  prac- 
ticable, however,  for  economic  and  hygienic  reasons. 

The  use  of  manufactured  deodorants,  of  which  there  are 
several,  is  more  desirable. 

Among  these  "facilol,"  made  by  the  tar  product  factory 
"Biebrich"  at  Biebrich  has  been  found  effective.  It  is  a  thin, 
brownish,  light  oil  with  a  specific  gravity  of  0.79.  It  forms  a 
coherent  film  of  oil  when  placed  upon  water,  which  closes  imme- 
diately if  broken  by  gas  bubbles  or  sudden  currents,  preventing 
the  escape  of  odors.  About  28  per  cent,  of  facilol  is  composed  of 
soluble  substances  of  the  phenol  group,  the  effect  of  which  is  to 
prevent  putrefaction  in  sludge  and  sludge  liquor.  The  eggs  and 
larvae  of  insects  are  also  killed  by  it,  while,  at  the  same  time,  the 
covering  prevents  the  insects  themselves  from  obtaining  their 
food. 

The  facilol  is  sprayed  upon  the  surface  of  the  sludge  immedi- 
ately after  its  entrance  into  the  basins  by  a  spraying  device. 
The  film  of  facilol  is  maintained  intact  by  subsequent  spraying 
at  long  intervals.  According  to  information  furnished  at  Frank- 
fort 0.11  to  0.18  gallons  of  facilol  per  square  yard  (0.5  to  0.8  1 
per  sq.  m.)  of  surface  will  suffice.  The  price  is  about  $2.15  per 
100  Ibs.  (20  m.  per  100  kg.).  This  mode  of  deodorizing,  there- 
fore, though  efficient,  is  rather  expensive. 

As  the  intensity  of  the  odors  increases  with  the  area  exposed  it 
might  be  well  to  put  the  sludge  to  be  dried  in  as  deep  pits  as 
possible  whose  bottoms  have  been  drained,  and  this  has,  in  fact, 
been  tried.  The  crust  of  dried  sludge  which  forms  at  the  top 
prevents  the  evaporation  which  assists  to  a  considerable  extent 
in  the  reduction  of  water.  Openings  in  the  crust  permit  the  air 
to  enter  but  a  short  distance.  In  consequence,  the  process  of 
drying  and  also  the  nuisance  of  foul  odors,  which  cannot  be 
prevented  by  subsequent  treatment  with  lime  or  peat,  last  for 
years. 

Only  when  natural  pits  exist,  as  at  Leipzig,  in  the  shape  of  an 
old  river  bed,  and  then  at  some  remote  point,  can  this  method  of 
drying  be  used.  Moreover,  the  difficulty  of  conveying  the  de- 
watered  sludge  partly  offsets  any  saving  consequent  to  dispens- 
ing with  an  artificial  drying  place. 

Odors  and  the  nuisance  of  flies  may  be  considerably  diminished 
by  the  means  above  mentioned  so  that  the  conditions  are  more 
tolerable  for  the  employees  at  the  plant. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    61 

In  general,  however,  it  is  preferable  to  remove  the  drying  place 
from  the  plant  when  the  neighboring  land  is  occupied,  unless  a 
method  to  be  described  later  be  adopted,  and  to  so  locate  it  that 
it  will  not  be  a  nuisance  to  the  neighborhood.  Land  of  little 
value  can  be  used  for  this  purpose  and  may  be  correspondingly 
extensive.  Sludge  conduits  3000  ft.  (1  km.)  or  more  in  length 
have  been  used  for  this  purpose  in  Germany.  Closed  pipes  should 
be  used  preferably.  In  selecting  a  place  the  prevailing  wind 
should  be  considered — that  it  does  not  blow  from  the  drying  beds 
toward  built-up  areas. 

Drying  beds  for  septic  tanks  do  not  require  the  same  restric- 
tions, as  there  are  no  odors  where  the  sludge  is  properly  digested. 

The  time  required  for  drying,  and  consequently,  to  a  certain 
extent,  the  size  of  the  sludge  beds,  depends: 

1.  On  the  composition  of  the  sludge. 

2.  On  the  character  of  the  soil  or  the  construction  of  the  bottom 
of  the  basin. 

3.  On  the  atmospheric  conditions. 

4.  On  the  method  of  operation  of  the  sludge  basin. 

The  composition  of  the  sludge,  and  in  particular  the  amount  of 
moisture  contained,  determine  to  a  great  degree  the  length  of 
time  necessary  for  drying.  For  example,  1.3  cu.  yds.  (1  cbm.) 
of  sludge  containing  95  per  cent,  moisture  must  have  198  gallons 
(750  1.)  of  the  water  removed  before  obtaining  sludge  with  80 
per  cent,  moisture,  as  is  found  with  septic  tanks.  A  fine,  greasy 
sludge  gives  up  its  moisture  less  readily,  and  under  some  circum- 
stances is  very  difficult  to  dry;  while  a  thinner  and  less  compact 
sludge  has  the  opposite  characteristic  and  gives  up  its  moisture 
easily.  The  granular,  fluid  septic  tank  sludge,  as  well  as  that 
from  lignite  treatment,  has  this  favorable  quality.  The  de- 
posited sludge  from  the  Kremer  apparatus  is  easily  de-watered 
as  it  contains  so  little  grease. 

A  basin  with  a  porous  base  may  be  of  less  size  than  if  compact. 
If  the  bottom  does  not  promise  free  percolation  it  should  be 
arranged  as  an  artificial  filter.  Care  should  then  be  taken  to 
clean  or  renew  the  covering  layer  frequently. 

As  evaporation  has  a  marked  effect  on  the  drying,  this  takes 
place  more  rapidly  in  summer.  A  sunny  or  windy  location  is 
also  favorable  to  drying. 

To  secure  the  best  results  drying  should  take  place  quickly 
on  an  ample  area.  The  sludge  should  therefore  be  distributed  in 


62  SEWAGE  SLUDGE 

thin  layers — about  6  to  10  in.  (15  to  25  cm.).  A  rapid  loss  of 
water  through  the  subsoil  occurs,  and  the  cracks  caused  by  drying, 
plainly  seen  in  Fig.  21,  permit  the  air  to  pass  to  the  underlying 
strata.  Sludge  shrinks  in  drying  to  from  2  to  3  to  1  to  2  its 
original  volume.  A  fresh  layer  can  then  be  admitted  on  the 
dried  layer. 

Sometimes  the  sludge  is  dried  directly  in  the  settling  tanks. 
These  must  then  be  thrown  out  of  service  for  some  time,  as  by 
this  method  evaporation  does  most  of  the  work.  The  time  used 
for  drying  lignite  sludge  in  this  way  at  Copenick  is  from  3  to  4 
weeks.  There  is  a  project  for  the  adoption  of  a  similar  method  at 
Neustrelitz  ("Mitteilung  d.  Kgl.  Priif.  Anstalt/;  Vol.  VI).  The 
size  of  the  necessary  basins  prevents  its  use  where  these  are  fixed 
or  where  there  is  insufficient  land.  Special  beds  for  this  purpose 
are  always  desirable  on  account  of  the  greater  rapidity  of  drying. 

Different  intervals  are  required  for  drying,  depending  upon  the 
different  conditions  mentioned  above.  As  already  stated,  the 
septic  tank  combines  the  most  favorable  of  these  conditions. 
The  length  of  time  required  for  drying  by  the  Emscher  Associa- 
tion, e.g.,  averages  5  days — with  favorable  weather  only  one  or 
two  days.  Sludge  taken  from  the  septic  tanks  at  Halberstadt  at 
intervals  of  about  8  weeks  requires  14  days  for  drying  in  good 
weather.  In  both  cases  it  is  received  in  thin  layers.  In  contrast 
to  these  are  those  plants  where  sludge  is  delivered  to  the  sludge 
basin  to  a  depth  of  2.3  to  3.3  ft.  (0.7  to  1.0  m.).  The  process  of 
drying  here  usually  takes  from  6  to  9  months. 

In  general,  normal  settled  sludge  with  about  90  per  cent,  water, 
requires  some  6  or  8  weeks  in  summer  and  6  months  in  cold 
weather. 

The  size  of  sludge  basins  can  be  estimated  from  the  amount 
accumulated  daily  and  its  average  time  for  drying,  allowing  a 
certain  time  for  the  removal  of  the  dried  material.  Both  factors 
are  subject  to  great  variation  with  different  processes  and  plants 
for  reasons  already  given,  so  that  an  estimate  based  upon  these 
figures  would  have  no  practical  value.  As  a  guide  for  the  area 
required  for  drying,  the  following  table,  showing  the  size  at 
different  plants,  is  given;  for  a  great  nuisance  may  result,  as  has 
been  shown  by  examples,  where  these  are  made  too  small,  while 
too  large  an  allowance  results  in  too  great  a  cost,  especially  in 
the  neighborhood  of  high-priced  city  property. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    63 


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64  SEWAGE  SLUDGE 

The  size  of  drying  beds  is  governed  by  the  amount  of  sludge 
which  accumulates  during  the  time  required  for  drying,  the 
reasons  for  this  time  being  given. 

There  is  a  great  difference,  therefore,  between  the  size  required 
for  plain  sedimentation  and  that  required  for  septic  treatment. 
Imhoff  gives  in  Vol.  VII  of  "Mitteilung  der  Kgl.  Versuchsan- 
stalt"  the  following  rules  for  size,  based  upon  his  observations: 

For  septic  tank  sludge  275  sq.  yds.  per  cubic  yard  (300  qm. 
per  cbm.)  daily  of  sludge  received,  and  therefore  for  an  amount 

about  0.1  qm.)  per  capita. 

For  sludge  from  plain  sedimentation  458  sq.  yds.  per  cubic  yard 
(500  qm.  per  cbm.)  daily  of  sludge  received,  and  therefore  with 
.0016  cu.  yd.  (1.21.)  of  sludge  per  capita  daily,  0.72  sq.  yds. 

1.  2X500 


The  latter  values  agree  with  the  table,  while  the  former  appear 
too  high,  so  that  with  septic  sludge  about  183  sq.  yds.  of  drying 
surface  will  be  necessary  for  1  cu.  yd.  of  sludge  per  day  (200  qm. 

(0  3  X200 
Lr™r—  =0.06  qm.)  per  capita. 
lUUu 

This  shows  clearly  the  advantage  of  septic  tanks  as  compared 
with  plain  sedimentation,  both  on  account  of  the  smaller  amount 
of  water  contained  and  the  smaller  amount  to  be  evaporated. 

The  values  in  the  table  naturally  indicate  marked  differences 
in  different  cities.  This  is  due  in  part  to  the  different  methods 
of  treatment  and  handling  and  partly  to  the  fact  that  the  oper- 
ation of  drying  beds  where  space  is  limited  must  be  much  more 
intensive  than  where  the  opposite  conditions  obtain,  in  which 
case  the  sludge  remains  longer  than  necessary,  until  it  is  con- 
venient for  the  farmers  to  remove  it. 

An  estimate  of  the  size  of  sludge  lagoons  based  upon  the  daily 
flow  of  sewage  would  result  in  yet  greater  differences  and  is 
therefore  not  given. 

b.  DRYING  BY  FILTER  PRESSES 

De-watering  sludge  by  filter  presses  was  first  tried  in  England 
about  30  years  ago,  and  was  soon  in  general  use  there.  The 
large  volumes  of  sludge  which  resulted  from  chemical  precipita- 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    65 

tion,  then  in  general  use,  and  the  odors  from  these  plants  which 
were  especially  unpleasant  in  thickly  populated  districts,  led  to 
a  rapid  spread  of  this  method  of  drying.  In  default  of  other 
methods  its  disadvantages  were  willingly  overlooked. 

In  Germany,  also,  sludge  presses  are  found  almost  exclusively 
in  plants  where  chemical  precipitation  is  or  has  been  employed, 
except  in  the  lignite  process,  which  should  be  included  here. 

Filter  presses  consist  of  a  large  number  of  thin  cells,  usually 
about  2.3  to  3.3  ft.  (0.7  to  1  m.)  square  and  2  in.  (0.05  m.)  thick. 
According  to  the  design  of  the  separate  parts  which  compose 
these  cells  they  are  called  cell  presses  or  frame  presses.  In  the 
former  the  partition  plate  between  two  cells  is  provided  on  each 
side  with  rims  projecting  about  2  in.  (5  cm.)  (Fig.  23)  so  that  the 

aba 


FIG.  22. 
Frame  press. 


FIG.  23. 
Cell  press. 


edge  of  adjacent  plates  by  coming  in  contact  form  a  hollow  space 
between.  In  the  frame  presses  these  plates  are  of  uniform  thick- 
ness (a)  and  to  form  the  cells  a  frame  is  inserted  between  two 
plates  (b).  This  forms,  at  the  same  time,  the  narrow  walls 
of  the  cell  (Fig.  22) .  These  plates  or  partitions  in  the  cell  press 
are  provided  with  numerous  grooves  opening  below  into  a 
horizontal  hole  (c)  which  serves  as  a  channel  for  the  liquor 
pressed  out.  Over  these  plates  are  placed  sieves,  and  over  these 
filter  cloths  are  stretched,  or  possibly  the  latter  only  (d)  are 
provided,  so  that  in  filling  the  cells  with  sludge  the  surplus  water 
will  pass  through  the  cloth  and  run  down  the  partition  plates  in 
the  grooves.  It  then  leaves  the  press  through  the  ducts  (c). 
The  separate  plates  are  supported  by  lateral  projections  on 
horizontal  bars  and  are  pushed  together  by  hand.  The  water- 


66  SEWAGE  SLUDGE 

tight  contact  is  then  accomplished  by  means  of  a  screw  or  by  the 
plunger  of  a  hydraulic  press.  The  influent  channel  (e)  lies  either 
in  the  upper  edge  or  th*e  middle  of  the  plate.  In  the  former 
case,  the  sludge  can  be  introduced  at  several  points  by  flexible 
pipes,  while  in  the  latter  it  must  be  done  at  the  middle  of  the 
front  plate. 

In  frame  presses  the  filter  cloths  are  simply  hung  over  the 
entire  frame  so  that  they  pover  both  sides  and  are  clamped  at 
the  edge  by  the  adjacent  frame.  In  cell  presses,  on  the  con- 
trary, the  edges  of  the  cloths  laid  in  the  recess  between  the  plates 
must*  be  made  water-tight  in  a  special  way.  The  filter  cloths 
are  rapidly  destroyed  by  rotting,  which  is  most  active  at  the 
top  of  the  solid  plates,  as  the  damp  cloths  are  here  always  in 
contact  with  the  air.  Saturation  with  tar  at  this  place  pro- 
longs their  life,  which  is  generally  about  4  weeks. 

The  frames  may  be  made  of  iron  or  wood.  The  latter  is 
preferable  for  sludge  presses,  as  iron  rusts  easily  from  acids  that 
may  occur  in  the  sludge.  The  number  of  plates  varies,  but 
50,  each,  of  plates  and  frames  would  be  the  maximum. 

The  delivery  of  the  sludge  and  the  necessary  pressure  of  3  to  8 
atmospheres  for  pressing  can  be  provided  by  a  sludge  pump  or 
compressed  air.  The  sludge  pump  can  work  directly  on  the 
press,  as  e.g.,  at  Halle.  A  relief  valve  must  then  be  inserted  in 
the  sludge  press  pipe  which  permits  the  surplus  sludge  to  pass 
off  from  a  fairly  full  press,  where,  therefore,  only  a  small  amount 
of  sludge,  as  compared  with  the  volume  of  the  liquor  drained 
out,  can  be  received.  This  surplus  sludge,  as  well  as  that  remain- 
ing in  the  pipe  after  filling,  then  flows  into  a  special  sludge  well 
and  is  the  first  to  be  pumped  out  before  refilling.  During  the 
emptying  of  the  press,  which  lasts  from  10  to  30  minutes,  the 
delivery  of  sludge  from  the  well  must  cease. 

Otherwise  we  may  provide  a  sludge  receiver  (Spandau), 
which  is  filled  by  the  sludge  pump  while  discharge  takes  place 
by  air  pressure.  The  sludge  receiver  can  be  filled  by  suction 
and  the  contents  then  forced  out  into  the  press  by  air  pressure. 
This  method  permits  uninterrupted  operation  by  installing  two 
receivers,  side  by  side,  as  already  described  for  propelling  sludge. 
The  presses  can  be  operated  to  greater  advantage  by  the  use  of 
sludge  receivers.  Therefore  this  method  is  always  to  be  pre- 
ferred for  large  volumes  of  sludge  to  the  slower  method  of  filling 
the  presses  with  a  pump.  The  employment  of  a  stirring  device, 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    67 

particularly  with  long,  extended  receivers,  is  not  desirable  or 
necessary,  as  it  is  impossible  to  support  the  shaft  properly  by 
means  of  intermediate  bearings. 

To  empty  the  presses  [which  yield  an  average  of  about  2.6 
cu.  yds.  (2  cbm.)  of  sludge  cake  for  each  filling]  the  frames  are 
pushed  apart.  The  sludge  then  drops  into  a  tip-car  placed 
below,  or  into  a  channel.  The  contained  moisture  is  reduced  to 
about  50  or  60  per  cent.  Further  drying  in  the  air  should  be 
given  the  sludge  in  covered  chambers,  as  otherwise  it  decom- 
poses, especially  in  wet  weather.  If  allowed  to  lie  for  any 
length  of  time  in  the  open  it  is  well  to  cover  it  with  a  layer  of 
earth  or  sod  to  prevent  objectionable  odors. 

Emptying  presses  is  a  dirty  operation,  and  in  summer,  espe- 
cially, it  produces  very  foul  odors  which  can  only  be  prevented 
by  thorough  ventilation  of  the  press  chambers.  Water  under 
pressure  is  absolutely  necessary  for  rinsing  purposes.  Some- 
times the  filled  presses  are  allowed  to  stand  several  hours  after 
filling,  even  12  hours  in  Oberschoenenweide,  to  give  more  con- 
sistency to  the  sludge. 

As  settled  sludge  is  never  very  firm,  the  advantage  in  chamber 
presses  that  the  contents  fall  out  when  they  are  opened,  while 
frame  presses  require  subsequent  cleansing  by  hand,  is  offset 
by  the  difficulty  in  fastening  the  filter  cloths.  Frame  presses 
are  therefore  to  be  preferred  for  pressing  sludge. 

The  liquor  drained  from  the  press  is  very  putrescible,  and  to 
be  clarified  should  be  brought  to  the  influent  conduit  again  or 
should  be  used  in  irrigation. 

Sludge  from  domestic  sources  that  has  been  obtained'  by 
mechanical  processes  cannot  be  pressed.  A  large  part  of  the 
finest  particles  of  this  very  watery  sludge  passes  through  the 
filter  cloths,  robbing  the  sludge  cakes  which  remain  behind  of 
their  binding  medium,  so  that  when  the  press  is  opened,  the 
sludge  is  found  in  an  incoherent  mass.  Greasy  or  slimy  sludge 
may  also  prevent  pressing  by  clogging  the  cloths.  Sludge  from 
septic  tanks,  on  the  contrary,  can  often  be  pressed  without  any 
further  treatment,  if  it  has  not  lain  too  long  in  the  tank.  The 
same  is  true  of  sludge  from  the  lignite  process,  which  is  almost 
always  successfully  de-watered  by  the  filter  press.  Chemical 
precipitation,  however,  is  the  principal  treatment  for  which 
sludge  pressing  is  used,  as  the  precipitants  employed  render  the 
sludge  cakes  cohesive. 


68  SEWAGE  SLUDGE 

A  dose  of  lime  is  necessary  to  make  an  unfavorable  sludge 
capable  of  being  pressed.  8.4  Ibs.  per  cubic  yard  (5  kg.  per 
cbm.)  is  sufficient,  or  5  per  cent,  of  the  dried  material  in  the 
sludge.  This  addition  is  also  sometimes  necessary  after  chemical 
precipitation.  Of  the  English  cities,  Chorley  adds  8.4  Ibs., 
Blackburn  8.4  to  13.5  Ibs.,  Bury  10.1  Ibs.  per  cubic  yard  (5  kg.,  5 
to  8  kg.  and  6  kg.,  respectively,  per  cbm.)  to  the  sludge  obtained 
by  alumino  ferric  as  precipitant.  With  greasy  deposits  these 
amounts  must  often  be  increased.  At  Willesden  it  is  37  Ibs., 
at  Colchester  47  and  at  Ealing  even  84  Ibs.  per  cubic  yard  (22  kg., 
28  kg.  and  50  kg.  per  cbm.,  respectively),  in  order  to  secure 
hard  cakes,  which  can  then  be  incinerated. 

The  necessity  of  adding  some  substance  to  obtain  a  sludge  that 
can  be  pressed  has  helped  to  retain  the  use  of  chemical  precipita- 
tion in  England,  for  it  is  more  reasonable  to  use  lime  in  the 
clarification  process  than  to  add  it  merely  for  pressing.  The 
addition  of  lime  reduces  the  cost  of  pressing,  but  increases  the 
total  cost  of  clarification.  Dunbar  gives  an  excellent  example 
of  this  (Leitfaden  fiir  die  Abwassereinigungsfrage) .  In  Wim- 
bleden,  by  using  lime  and  iron  precipitants,  8.5  tons  of  sludge 
cake  per  million  gallons  of  sewage  cost  53.4  cts.  per  ton  (2  long 
tons  per  1000  cbm.  cost  2.51  M.  per  long  ton)  for  pressing,  or 
about  $4.50  per  million  gallons  (5  M.  per  1000  cbm.)  of  sewage. 
By  adding  lime  this  was  reduced  to  36  cts.  per  ton  (1.68  M. 
per  long  ton).  But  12.7  tons  per  million  gallons  of  sewage  (3 
long  tons  per  1000  cbm.)  of  pressed  cakes  were  obtained,  so  that 
the  cost  of  pressing  per  million  gallons  of  sewage  was  as  high 
as  before.  The  cost  of  the  additional  precipitant  was  included, 
in  addition  to  which  11/2  times  the  volume  of  sludge  was  ob- 
tained. The  questions  of  cost  and  increase  of  sludge  are  to  be 
considered  in  increasing  the  precipitant,  for  it  is  not  reasonable 
to  add  precipitants  merely  to  secure  sludge  which  can  be  pressed 
without  increasing  the  clarification. 

Sometimes  very  greasy  sludge  refuses  to  take  up  the  lime,  and 
de-watering  must  be  accomplished  in  some  other  way.  This  has 
been  the  case  in  several  English  cities. 

In  Frankfort-on-the-Main,  too,  the  filter  presses  have  been 
abandoned,  for  it  was  found  on  opening  the  presses  that,  even 
with  great  pressure,  only  the  layer  next  the  cloths  had  been 
de-watered  and  caked,while  the  middle  of  the  chamber  was  full 
of  liquid  sludge.  A  satisfactory  result  was  only  obtained  after 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    69 

adding  9.5  Ibs.  (4.3  kg.)  of  sulphate  of  alumina  while,  at  the 
same  time,  heating  the  press.  This  increased  the  cost  to  $1.00 
per  cubic  yard  (5.50  M.  per  T  cbm.)  of  sludge,  rendering  any 
practical  use  of  the  method  prohibitory. 

The  cost  of  pressing  1  ton  of  cakes  obtained  from  5.8  cu.  yds. 
of  watery  sludge  in  English  plants,  including  interest  and  sinking 
fund  charges,  is,  according  to  Schiele,  $0.42  1/2  to  $1.28  or  an 
average  of  about  85  cts.  (1  long  ton  from  5  cbm.  sludge,  2.00  to 
6.00  M.  or  an  average  of  about  4  M.).  Reichle  and  Thiesing 
give  for  the  same  amount  about  $0.49  (2.30  M.  for  1  long  ton) 
as  the  cost  of  pressing  under  German  conditions,  assuming  the 
cost  of  the  press  at  $1785  (7500  M.)  and  amortization  at  5  per 
cent.  But  an  extra  dose  of  lime  is  not  included.  This  last 
estimate  assumes  the  most  favorable  operation  of  the  presses, 
so  that  the  price  per  ton  of  pressed  cake  in  Germany  may  be 
taken  at  about  63  1/2  cts.  to  85  cts.  (3  to  4  M.  per  long  ton). 

For  the  addition  of  lime,  usually  in  the  form  of  milk  of  lime,  in 
England,  a  tank  is  inserted  between  the  sludge  pit  and  the  sludge 
holder  by  the  presses,  in  which  the  sludge  remains  quiescent  for 
a  short  time,  sometimes  for  several  days,  after  the  dose  of  lime 
has  been  added. 

With  modern  plants  it  is  seldom  necessary  to  resort  to  presses 
for  de-watering.  The  advantages  of  the  short  time  and  the 
limited  space  required  for  drying  as  compared  with  drying  in  the 
air,  are  offset  by  the  greater  cost.  The  nuisance  due  to  foul  odors 
is  reduced,  but  not  entirely  eliminated,  and  the  workmen  have  to 
come  into  contact  with  the  sludge  to  a  considerable  extent. 

The  lignite  process  is  accompanied  by  a  minimum  of  offensive 
odors  and  as  it  is  more  favorable  for  pressing  on  account  of  the 
addition  of  chemicals,  this  method  has  been  retained  here. 
These  plants  are  particularly  adapted  for  use  in  thickly  populated 
districts  on  account  of  their  arrangements,  such  as  the  complete 
enclosure  of  the  sewage  during  clarification  in  towers,  etc.  The 
greatest  importance  naturally  is  attached  to  a  quick  removal 
of  the  water,  the  occupation  of  but  little  room,  and  a  prompt 
disposition  of  the  sludge,  which  is  in  this  case  effected  by  in- 
cineration. 

c.  DE-WATERING  SLUDGE  BY  CENTRIFUGAL  MACHINES 

The  disadvantages  of  filter  presses  led  to  experiments  with 
other  mechanical  appliances  for  de-watering  sludge.  Drying  by 


70  SEWAGE  SLUDGE 

centrifugal  machines,  such  as  are  used  in  laundries  and  bleach- 
eries,  seemed  to  promise  the  best  results. 

The  water  that  is  drawn  off  by  centrifugal  force  from  the 
material  to  be  dried  passes  out  through  the  sieve-like  sides  of  the 
rotating  drum. 

This  process  did  not,  however,  give  the  hoped-for  results.  In 
the  first  place  the  finer  particles  of  sludge  were  thrown  out 
through  the  perforated  or  cloth-covered  sides.  In  the  second 
place  the  heavier  portions  were  thrown  against  the  sides  of  the 
drum  by  the  centrifugal  force  so  compactly  that  after  a  while  no 
more  water  could  penetrate  it. 

Sludge  is  so  disintegrated  by  the  centrifugal  action  that  the 
heavier  portions — the  mineral  ingredients — go  to  the  outside. 
The  organic  materials  come  next  in  concentric  layers,  while  the 
liquid  portion  remains  in  the  middle  with  the  grease  on  its  surface. 

This  behavior  of  the  sludge  led  to  the  construction  of  a 
centrifugal  machine  similar  to  those  used  in  milk  separators, 
with  a  solid  shell,  the  water  being  led  off  as  by  a  siphon.  This 
method  required  a  long  time,  as  was  demonstrated  by  experi- 
ments at  different  places  (Spandau,  Frankfort-on-the-Main, 
Mannheim.  In  Spandau  with  sludge  from  the  lignite  process, 
30  to  45  minutes).  Besides  the  solid  ring  of  sludge  collected  at 
the  shell  had  to  be  cleaned  out  by  hand  with  a  spoon-shaped 
instrument  at  the  end  of  each  period  of  operation. 

In  experiments  made  with  such  a  machine  at  Chemnitz, 
furnished  by  the  Haubold  machine  works  of  that^city,  3  cu.  yds. 
(2.5  cbm.)  of  sludge  was  de-watered  in  about  10  minutes  from 
90  per  cent,  to  45  per  cent,  or  60  per  cent,  reduction  of  moisture 
as  described  in  Schmeitzner's  "Clarification  of  Sewage."  It 
required  6  h.  p. 

An  attempt  was  made  to  circumvent  the  dirty,  slow  and  un- 
healthy work  of  cleaning,  which  prevented  its  adoption  in  large 
plants  and,  in  addition,  lessened  the  efficiency  of  the  machine,  by 
introducing  a  bottom  which  could  be  lowered  and  the  automatic 
ejection  of  the  sludge.  The  cost  remained  high,  however,  on 
account  of  the  length  of  time  required,  although  as  to  the  quality 
of  the  product,  the  results  secured  were  satisfactory.  The 
addition  of  1  per  cent,  of  lime  to  bind  the  grease,  thus  improving 
the  material  to  be  ejected  as  well  as  the  liquid  effluent,  was  found 
useful. 

The  entire  process  can  only  be  of  practical  use  if  the  wet  sludge 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    71 

can  be  brought  in  and  the  dried  sludge  removed  automatically 
without  stopping  the  machine,  while  reducing  the  time  required 
for  the  work. 

These  conditions  have  been  fulfilled  by  the  Hanover  Machine 
Co.,  formerly  G.  Egestorff,  at  Hanover-Linden,  in  a  centrifugal 
machine  placed  on  the  market  as  the  Schaefer-ter  Meer  System 
(German  Patent)  which  was  constructed  after  many  experiments 
by  Director  ter  Meer  in  conjunction  with  City  Engineer  Schaefer, 
Frankfort-on-the-Main. 


FIG.   26 
FIGS.  24,  25  and  26. — Centrifugal  machine. 

Two  of  these  machines  have  been  in  use  in  Harburg,  four  in 
Hanover  and  six  in  Frankfort. 

As  this  system  has  been  found  of  practical  value  for  some  time, 
we  will  consider  its  operation  in  more  detail. 

The  part  of  the  apparatus  which  collects  the  sludge  while 
throwing  off  the  remainder,  consists  af  six  radial  chambers,  having 
a  rectangular  cross-section  (Figs.  24  and  25),  one  radial  side  of 
which  is  formed  by  a  sieve-like  plate.  This  has  slits  0.4  in. 


72  SEWAGE  SLUDGE 

(10  mm.)  long  and  .016  to  .024  in.  (0.4  to  0.6  mm.)  wide.  The 
chambers  are  closed  on  the  inside  and  outside  by  slide  valves  and 
have  a  capacity  of  about  1  to  10  cu.  ft.  (3  1.)  each. 

The  process  of  ejection  is  as  follows:  the  wet  sludge  flows  from 
a  receiver  placed  at  a  higher  elevation  through  the  central  inlet 
pipe  into  the  chambers  while  these  are  in  rotary  motion.  The 
inner  valves  are  thereby  opened  and  the  outer  valves  closed. 
The  heavier  particles  of  sludge  are  now  thrown  against  the  outer 
part  of  the  chamber,  by  which  action  the  water  is  forced  inward, 
partly  by  the  compression  of  the  mass  and  partly  because  of  its 
lighter  weight,  and  flows  out  through  the  sieves  to  the  surround- 
ing water  chamber.  From  this  it  passes  through  short  pipes  into 
a  circular  gutter  and  thence  to  the  outlet  (Fig.  24) . 

Sludge  takes  the  place  of  the  water  thrown  off  until  the  chamber 
is  quite  filled  with  the  de-watered  material.  The  inner  ring 
valve  is  then  closed  preventing  the  admission  of  any  more  sludge, 
and  the  entire  mass  is  then  whirled  around  for  a  period  depending 
upon  the  composition  of  the  sludge.  The  outer  valves  then 
open  and  the  dried  sludge  is  thrown  out  by  the  centrifugal  force. 
It  flies  against  the  wall  of  the  shell,  being  loosened  up  and  dis- 
integrated, and  then  falls  down  and  out  of  the  apparatus  onto  a 
belt  conveyor. 

Between  the  outer  valves  and  the  wall  of  the  shell  is  a  circular 
movable  impact  wall.  This  serves  to  intercept  any  water  that 
might  escape  during  the  process  of  ejection  through  the  outer 
valves,  which  may  not  be  tight,  keeping  it  away  from  the  dried 
sludge  and  the  conveyor.  It  is  raised  a  short  time  before  the 
valve  is  opened,  to  leave  the  way  clear  for  the  sludge  to  be 
thrown  out. 

After  the  chambers  have  been  emptied,  the  outer  valves  close 
again,  and  as  the  inner  ones  open,  the  chamber  is  again  filled 
with  wet  sludge  and  the  operation  is  repeated. 

The  separate  movements  are  quite  automatic  and  even  the 
operation  of  the  valves  is  governed  through  two  cylinders  by 
means  of  press  oil  (Fig.  26)  which  is  stored  in  an  accumulator  by 
means  of  a  special  pump.  This  also  serves  as  a  bearing  to 
support  the  pressure  of  the  centrifugal  drum  shaft.  The  ad- 
mission of  the  press  oil  into  the  moving  cylinder  is  regulated  by  a 
regulating  valve. 

The  sieve  surfaces  are  cleaned  partly  by  the  passing  through 
of  the  dried  material  when  being  ejected,  partly  by  special 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     73 

scrapers   which   are   operated   mechanically  by  the   regulating 
apparatus. 

.     The  ejecting  drum  makes  750  r.  p.  m.,  corresponding  to  a 
circumferential  velocity  of  105  ft.  (32  m.)  per  second. 

The  length  of  a  working  period  in  the  experiments  made  by 
Reichle  and  Dr.  Thiesing  at  the  Harburg  plant  ("Mitteilung  der 
Kgl.  Priifungs-Anstalt,"  Vol.  X)  averaged  2.5  minutes,  or  3.5 
minutes  at  the  most.  In  the  plant  at  Hanover  the  standard 
time  is  1.5  minutes,  though  this  may  be  increased  to  5  minutes 
when  the  sludge  is  very  slimy. 

In  order  to  dry  successfully  by  centrifugal  force,  the  contents 
of  the  sludge  should  be  somewhat  heavier  than  water,  as  the 
entire  action  depends  upon  the  stronger  repulsion  due  to  the 
greater  specific  gravity  of  the  material.  The  fine  particles  of 
sludge  separate  themselves  out  from  the  sewage  and  adhere  to 
this  heavier  material  on  account  of  their  sticky  nature.  Fresh 
sludge  is  therefore  easier  to  work  over  than  that  which  has 
decomposed,  as  the  proportion  of  fine  particles  has  increased  in 
the  latter  by  disintegration. 

The  efficiency  of  a  centrifugal  machine  varies  according  to  the 
composition  of  the  raw  sludge.  2.6  to  5.2  cu.  yds.  (2  to  4  cbm.) 
of  raw  sludge  can  be  de-watered  in  an  hour  when  20  to  33  gallons 
(75  to  125  1.)  of  wet  sludge  is  admitted  to  the  chamber  at  each 
filling. 

In  the  experiments  at  Harburg  1  cu.  yd.  of  raw  sludge  with 
about  92  per  cent,  of  moisture  yielded  an  average  of  294  Ibs. 
of  ejected  sludge  (175  kg.  per  cbm.)  or  634  Ibs.  (287.5  kg.)  per 
machine  per  hour.  This  contained  69.7  to  74.2  per  cent,  of 
moisture  and  the  dried  material  contained  a  somewhat  larger 
proportion  of  mineral  matter  than  the  raw  sludge,  but  much 
less  grease  (8.5  per  cent,  instead  of  14.2  per  cent.). 

The  efficiency  of  the  centrifugal  action,  i.e.,  the  ratio  of  the 
actual  volume  of  ejected  sludge  to  the  dried  material  in  the  raw 
sludge,  computed  on  the  basis  of  the  water  contained  in  the' 
ejected  sludge,  ranged,  in  the  Harburg  experiment,  46.3  to  69.8 
per  cent.,  averaging  60  per  cent.  According  to  experiments  at 
Frankfort  and  the  above  observations  at  Harburg,  this  figure 
should  be  somewhat  greater.  It  should  preferably  be  taken 
somewhat  greater  because  the  amount  of  dissolved  material, 
which  will  inevitably  reach  the  outlet,  must  be  deducted  from 
the  dried  material  in  the  raw  sludge. 


74 


SEWAGE  SLUDGE 


Fully  60  per  cent,  of  the  dried  material  contained  in  sludge 
is,  therefore,  removed  by  the  centrifugal  action,  while  about 
40  per  cent,  is  returned  to  the  clarification  plant  by  the  effluent 
and  must  be  treated  again.  The  volume  of  accumulated  sludge 
is  thus  increased  as  well  as  the  concentration  of  the  drainage 
water,  diminishing  somewhat  the  efficiency  of  the  clarification 
plant. 

The  ejected  sludge  contains  about  2  to  3  of  the  mineral 
matter  and  1  to  3  of  the  organic  matter  of  the  raw  sludge  as  the 
following  table,  compiled  from  observations  at  Harburg,  indicates: 

TABLE  SHOWING  RESULT  OF  CENTRIFUGAL  ACTION  ON: 

a.   One  Cubic  Yard  of  Raw  Sludge 


1 
Total  weight        Water 

Total  dried 

Ash 

Organic 

Grease 

Ibs.                   Ibs. 

material  Ibs. 

Ibs. 

matter  Ibs. 

Ibs. 

Raw  sludge  

1720 

1584 

134 

29.4 

104.8 

19.1 

Ejected  sludge..  .  . 

295 

214 

81 

20.1 

61.0 

6.9 

Effluent  

1423 

1370 

53 

4.7 

48.1 

b.  One  Cubic  Meter  of  Raw  Sludge 


Total  weight 

Water 

Total  dried 

Ash 

Organic 

Grease 

kg. 

kg. 

material  kg. 

kg. 

matter  kg.        kg. 

Raw  sludge  

1019 

939.6 

79.4 

17.4 

62.0 

11.3 

Ejected  sludge..  .  . 

175 

126.6 

48.1 

11.9 

36.2 

4.1 

Effluent  

844 

812.7 

31.3 

2.8 

28.5 

The  effluent  contained  on  an  average  3.7  per  cent,  of  dried 
material,  composed  of '9  to  10  organic  and  1  to  10  mineral  matter. 

The  large  amount  of  organic  matter  renders  it  very  putrescible, 
producing  foul  odors  even  during  its  separation.  At  Harburg 
it  is  returned  to  the  main  sewer  and  mixed  with  the  sewage.  At 
Hanover,  on  the  contrary,  it  is  conveyed  to  two  tanks  which  have 
been  emptied.  These  are  first  filled  with  the  liquid  which  has 
been  thrown  off,  which  is  then  carried  beyond  in  the  usual  manner. 

If  the  sewage  receives  further  treatment  on  irrigation  fields 
or  in  contact  beds  the  centrifuge  effluent  can  be  treated  with  it. 
In  large  plants  there  may  be  some  question  of  treating  it  in  septic 
tanks;  but  where  there  is  abundant  water  for  dilution  it  may  be 
discharged  into  it  directly. 


TREATMENT  AND   UTILIZATION  OF  SLUDGE     75 


76  -SEWAGE  SLUDGE 

A  sludge  holder  constructed  of  plate  iron  or  reinforced  concrete 
(Fig.  27,  upper  right  hand)  and  provided  with  a  stirring  device 
should  be  placed  above  every  two  centrifugal  machines,  so  that 
the  material  received  may  be  as  uniform  as  possible.  The  holders 
should  be  large  enough  to  contain  all  the  sludge  accumulated  in  a 
day,  so  that  the  tanks  can  be  emptied  of  sludge  independently 
of  the  centrifugal  machine  and  can  be  ready  for  use  again  in  the 
shortest  possible  time. 

A  screen  of  about  .4  in.  (10  mm.)  mesh  should  be  placed  before 
the  sludge  holder  in  case  none  has  been  provided  before  the  tank 
or  at  the  pump  well,  to  intercept  any  coarse  material  which 
might  interfere  with  the  operation. 

The  dried  sludge  falls  on  to  a  belt  which  passes  below  the 
centrifugal  machine,  and  at  Hanover,  for  example,  is  brought  to 
an  elevator  (Fig.  27  in  the  background)  which  carries  the  sludge 
to  a  reservoir  similar  to  a  silo.  It  has  been  observed,  however, 
that  the  sludge  forms  a  compact  mass  in  spite  of  the  very  steep 
slope  of  the  bottom,  and  can  only  be  discharged  into  the  car  below 
by  manual  assistance.  When  the  plant  was  visited  the  sludge 
was  allowed  to  fall  through  a  gate  in  the  bottom  directly  into  an 
enclosed  car  for  its  conveyance,  holding  about  2  cu.  yds.  (1.5  cbm.) 
It  is  advisable,  however,  to  omit  the  elevator,  which  requires 
considerable  power,  and  to  raise  the  sludge  the  short  distance  to 
the  top  of  the  car  by  the  required  inclination  of  the  belt  conveyor. 
The  introduction  of  a  small  hopper  which  can  be  closed  is  of 
advantage  in  order  to  hold  the  sludge  while  the  cars  are  being 
changed.  In  certain  cases  the  car  can  be  placed  directly  under 
the  centrifugal  machine. 

The  whole  plant  will  require  about  12  h.  p.  per  machine. 

The  centrifuge  alone  will  require  about  7.2  h.  p. 

If  a  suction  producer  gas  plant  is  employed  for  power,  costing 
about  1.2  cts.  (5  pfg.)  per  h.  p.  hour,  the  cost  of  operating  a 
centrifugal  machine  would  be  8.6  cts.  (36  pfg.)  per  hour;  or,  with 
electricity  at  1.2  cts.  (5  pfg.)  per  kw.  hour,  only  7.6  cts.  (32  pfg.). 
This  would  correspond  to  a  cost  for  current  of  5.13  cts.  per 
cubic  yard  (0.28  M.  per  1  cbm.)  of  raw  sludge  or  35.2  cts.  per 
ton  (1.63  M.  per  1000  kg.)  of  the  product. 

The  ejected  sludge  is  of  a  loose,  crumbly  consistency  and  conse- 
quently dries  readily.  Its  weight  is  1520  Ibs.  per  cubic  yard 
(900  kg.  per  cbm.).  The  tendency  to  putrefy  is  comparatively 
slight  if  it  is  sheltered  from  rain  and  sunshine,  but  it  is,  naturally, 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     77 

not  entirely  done  away  with  as  there  still  remains  much  organic 
matter  that  is  not  fully  digested.  When  collected  in  large  heaps 
its  temperature  rises  and  it  becomes  more  compact. 

The  centrifugal  process  reduces  the  volume  of  the  sludge  to 
about  1/6  of  the  original  amount,  when  the  product  contains 
about  55  to  70  per  cent,  of  moisture. 

The  apparatus  requires  but  little  attention  as  the  filling  and 
emptying  are  automatic,  so  that  one  attendant  suffices  for  two 
machines.  He  does  not  come  into  direct  contact  with  the  sludge, 
as  the  machine  is  entirely  enclosed. 

In  this  way  all  foul  odors  are  avoided,  especially  as  the  process 
requires  the  sludge  to  be  as  fresh  as  possible. 

Drying  is  rapid  and  the  sludge  is  soon  in  favorable  condition 
for  further  manipulation. 

The  only  drawback  is  the  high  cost  of  the  plant  and  its  opera- 
tion. The  cost  of  the  centrifugal  apparatus  with  the  stirring 
devices  and  the  oil  pressure  pump  is  about  $5230  (22,000  M.) 
with  an  additional  $240  (1000  M.)  for  the  motor.  Estimating 
5  per  cent,  for  amortization  and  the  price  for  electric  current 
already  given,  and  we  have  as  the  expense,  including  wages, 
polishing  material  and  reserve  sieves,  with  a  maximum  use  of  the 
apparatus,  62  cts.  per  ton  (2.87  M.  per  1000  kg.)  of  the  product 
for  drying,  or  fully  10  cts.  per  ton  (50  pfg.  per  1000  kg.)  more 
than  with  filter  presses  under  similar  conditions.  (See  page  69.) 

The  cost  of  de-watering  is  naturally  least  with  drying  in  the 
air,  especially  as  the  cost  of  the  plant  can  be  greatly  reduced  by 
a  simple  construction  of  the  drying  beds.  And  although,  as 
estimated  by  Reichle  and  Thiesing,  drying  with  filter  presses 
may  be  cheaper  under  similar  conditions,  this  is  offset  by  the 
necessity  of  adding  chemicals  to  the  sewage,  which  is,  as  a  rule, 
not  necessary  in  order  to  obtain  a  sufficiently  clear  effluent. 

The  centrifuging  of  sludge  is  particularly  advantageous  in  the 
case  of  simple  sedimentation,  where  it  is  desirable  to  secure  a 
rapid  reduction  of  moisture  or  where  the  establishment  of  drying 
beds  is  not  feasible,  for  lack  of  room  or  other  reasons. 

d.  OTHER  METHODS  OF  REDUCING  THE  WATER  IN  SLUDGE 

Some  methods  will  be  alluded  to  here  which  are  employed 
for  merely  a  slight  reduction  of  the  moisture  preparatory  to 
further  drying,  or  which  have  not  yet  passed  beyond  the  ex- 
perimental stage. 


78  SEWAGE  SLUDGE 

A  part  of  the  water  may  be  drained  off  by  letting  the  sludge 
settle  again  and  drawing  off  the  roily  water  which  has  separated 
out  above  the  sludge.  This  can  be  accomplished  in  sludge  wells 
or  in  special  sludge  holders,  such  as  are  inserted  before  filter 
presses  for  mixing  the  lime,  for  example.  The  reduction  of 
water  is  naturally  slight,  and  is  only  worth  considering  with  a 
very  watery  sludge,  such  as  is  obtained  by  movable  contrivances 
for  drawing  it  off  under  water.  On  the  other  hand,  it  is  unreason- 
able to  construct  special  plants  for  this  purpose,  especially  as  the 
very  dirty  turbid  liquid,  on  account  of  the  necessity  of  sub- 
jecting it  to  further  treatment,  is  detrimental  to  the  operation  of 
the  plant.  Moreover,  the  settled  sludge  should  not  be  stored 
without  some  special  reason,  as  the  freshest  possible  sludge  is 
the  best  for  subsequent  drying  by  mechanical  means. 

It  may  be  considered,  however,  where,  as  in  Allenstein,  a 
vacuum  receiver  can  take  in  the  whole  of  the  day's  supply  at  one 
filling.  This,  coming  from  wells,  remains  in  the  receiver  one 
day.  The  turbid  water  which  separates  out  is  then  drawn  off 
through  faucets  at  different  heights  in  the  receiver,  before  the 
sludge  is  propelled  further.  This  results  in  a  desirable  re- 
duction of  volume,  especially  when  it  is  conveyed  to  the  fields 
in  a  wet  condition. 

As  has  been  several  times  mentioned,  sludge  loses  a  part  of  its 
water  during  septicization,  and  acquires  a  more  favorable  char- 
acter for  further  drying  in  the  air.  The  aim  should  therefore 
be  to  convey  the  sludge  from  the  sedimentation  tanks  to  special 
digestion  chambers,  in  some  cases  with  admixture  of  a 
portion  of  the  unclarified  sewage  (Skegness,  Eng.).  This  is 
done,  e.g.,  at  Columbus  with  the  sludge  from  grit  chambers  and 
sedimentation  tanks.  It  is  only  advantageous  where  contact 
beds  or  artificial  or  natural  sand  filters  which  are  already  in- 
stalled for  the  rest  of  the  plant,  can  be  used  to  purify  the  effluent. 
Should  these  devices  be  installed  and  operated  merely  for  the 
digestion  of  the  sludge,  the  cost  would  be  out  of  proportion  to 
the  comparatively  slight  improvement  due  to  the  digestion. 

By  leaving  it  to  digest  the  sludge  may  be  stored  up  for  such 
times  as  it  is  required  for  fertilizing  the  land. 

A  certain  amount  of  digestion  also  takes  place  by  drying  in  the 
air,  as  indicated  by  the  generation  of  gas. 

This  favorable  alteration  in  sludge  is  readily  brought  about 
in  the  Emscher  tank,  already  mentioned,  by  combining  the 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     79 

settling  tank  with  the  digestion  chamber,  and  without  obtaining 
a  putrescent  effluent  or  being  annoyed  by  disagreeable  smells. 
The  gases  of  decomposition  contain  only  traces  of  sulphuretted 
hydrogen,  being  chiefly  composed  of  methane  and  carbonic  acid, 
and  are  therefore  almost  odorless. 

The  decomposition  here  differs  favorably  from  that  in  septic 
tanks  without  a  current,  as  is  usually  the  case  with  those  for  the 
digestion  of  sludge,  by  its  greater  intensity.  The  reason, 
according  to  Spillner  (Ges.  Ing.,  1909)  is  probably  that  fresh 
sludge  is  constantly  admitted  and  so  there  is  no  lack  of  bacteria 
and  their  nourishment.  At  the  same  time,  with  the  frequent 
removal  of  the  sludge,  and  in  exchange  for  the  fresh  sludge,  the 
disintegrated  product,  which  is  harmful  to  bacteria,  is  removed. 
The  septic  chamber  is,  moreover,  always  in  operation,  and  the 
disadvantage  of  receiving  undigested  sludge  when  sludge  is 
drawn  off  is  obviated.  This  occurs  if  the  septic  tank  is  not 
allowed  to  rest  for  several  weeks  after  shutting  off  the  inflowing 
sewage. 

A  very  different  process  from  any  mentioned  is  the  patent 
electro-osmose  method  of  Count  von  Schwerin,  with  which 
exhaustive  experiments  have  been  made  in '  the  drying  and 
utilization  of  sludge  at  Frankfort-on-the-Main. 

In  this  process  the  liquid  molecules,  by  osmosis,  pass  to  the 
cathode,  while  the  solid  particles  collect  at  the  anode,  when  an 
electric  current  is  passed  through  a  mass  of  sludge.  The  sepa- 
rated water  at  the  cathode  is  then  drawn  off.  The  colloids,  which 
form  a  large  part  of  the  sludge  and  which  render  drying  by 
mechanical  means  difficult  on  account  of  their  slimy  consist- 
ency, are  shriveled  up  by  the  electric  current,  facilitating  their 
separation. 

The  apparatus  consists  of  frames,  one  side  of  which  is  enclosed 
by  a  brass  sieve  which  forms  the  cathode,  and  the  other  by  a 
metal  plate,  usually  of  lead,  which  forms  the  anode.  The  in- 
side chamber  is  filled  with  sludge  and  the  anode  plate  is  brought 
near  the  cathode.  The  anode  plate  approaches  the  cathode  as 
the  volume  is  lessened  by  the  drawing  off  of  the  water  at  the 
cathode. 

Electrolytic  disintegration  is  brought  about  simultaneously  by 
the  electrolite  contained  in  the  sludge,  so  that  the  cathode  water 
is  alkaline,  while  at  the  anode  the  reaction  is  slightly  acid. 

The  current  used  in  this  method,  which  is  not  yet  past  the 


80  SEWAGE  SLUDGE 

experimental  stage,  is  rather  high,  but  not  so  great,  according  to 
Tillmans,  as  to  render  it  impracticable. 

Artificial  drying  over  a  fire  is  entirely  out  of  the  question  for  wet 
sludge  for  reasons  of  economy,  on  account  of  the  great  volume  to 
be  converted  into  steam;  but  it  has  been  tried  with  sludge  made 
somewhat  firm  by  air  drying  or  by  filter  pressing. 

The  cheapest  way  of  doing  this  is  by  making  briquettes  of  the 
solid  sludge  in  brick  presses,  which  are  then  dried  on  shelves 
under  a  roof,  and  in  from  2  to  8  weeks,  according  to  the  weather, 
become  sufficiently  hard  to  be  transported  without  special  pre- 
cautions. These  sludge  bricks  can  then  be  ground  up  and  used 
as  a  fertilizer. 

The  process  of  artificial  drying  consists  either  in  carrying  the 
sludge  on  a  belt  conveyor  through  a  heated  chamber,  or  it  is 
brought  to  a  current  of  hot  air,  by  an  enclosed  worm  conveyor, 
in  a  slowly-revolving  iron  drum.  The  foul  gases  which  arise  are 
led  under  the  fire  to  render  them  inoffensive. 

The  value  of  the  material  as  a  fertilizer  does  not  compare  with 
the  cost,  especially  of  the  coal,  as  will  be  shown  later;  so  that  such 
a  method  is  quite  impracticable,  aside  from  the  disagreeable  and 
unhygienic  features  of  the  work  due  to  the  odors  created,  and 
repeated  contact  with  the  sludge. 


CHAPTER  V 
UTILIZATION  OF  SLUDGE 

Ever  since  the  impure  matter  has  been  separated  from  sewage 
in  the  form  of  sludge  by  clarification  plants,  suggestions  and 
experiments  of  various  kinds  have  been  made  toward  the  most 
complete  and  profitable  recovery,  so  far  as  possible,  of  its  more 
valuable  constituents. 

The  following  conditions  should  be  fulfilled  in  any  process 
leading  to  the  utilization  of  sludge: 

1.  The  operation  should  take  as  little  time  as  possible,  and 
there  should  be  a  complete  removal  of  injurious  ingredients. 

2.  No  physical  harm  or  inconvenience  should  be  permitted  to 
come  to  the  workmen  or  to  the  neighborhood  of  the  works. 

3.  The  more  valuable  materials  in  the  sludge  should  be  ex- 
tracted or  recovered  to  the  fullest  extent. 

4.  The  process  should  be  economical — i.e.,  the  cost  of  operation 
should  not  be  greater  than  is  justified  by  the  anticipated  benefits. 

1.  Hygiene  demands  that,  on  account  of  the  nature  of  sludge, 
the  filth  obtained  from  sewage  should  not  be  stored  up  or  sub- 
jected to  long  drawn  out  manipulation,   especially  when  the 
previous  treatment  is  unobjectionable.     The  objectionable  sub- 
stances should  be  removed  or  altered  by  the  operation,  so  that 
the  final  product  is  inoffensive. 

2.  The  immediate  contact  of  the  workmen  with  the  sludge 
should  be  avoided  here,  as  in  its  removal  from  the  tanks  and  its 
drying,  for  fear  of  infection.     The  generation  of  gases  and  dis- 
agreeable smells  in  the  utilization  of  sludge  may  result  in  a 
nuisance  to  the  neighborhood,  as  has  been  mentioned  in  connec- 
tion with  drying. 

3.  As  the  amount  of  the  more  valuable  materials  in  sludge  is 
small,  so  that  large  quantities,  as  compared  with  the  amount  of 
the  product,  must  be  handled,  economy  demands  the  greatest 
possible  recovery  without  any  waste.     It  is  to  the  general  interest 
that  none  of  the  material  representing  any  considerable  value  in 
the  large  volume  of  sludge  collected  from  many  sources  should 

6  81 


82  SEWAGE  SLUDGE 

be  lost,  especially  in  view  of  the  successful  efforts  that  have  been 
made  in  all  branches  of  human  endeavor  to  recover  that  which 
was  formerly  considered  worthless. 

4.  The  hope  of  securing  a  revenue  from  sludge  utilization 
equal  to  the  cost  of  operation,  or  of  making  it  a  profitable  under- 
taking, has  long  been  destroyed,  at  least  with  city  sewage. 

This  is  easily  understood  when  one  considers  that  in  a  city  of 
50,000  inhabitants  with  an  output  of  2  million  gallons  (7500 
cbm.)  of  sewage  per  day,  perhaps  58.5  cu.  yds.  (45  cbm.)  of 
sludge,  90  per  cent,  moisture,  or  5.9  cu.  yds.  (4.5  cbm.)  of  dried 
material  are  recovered,  of  which  possibly  2.6  to  3.3  cu.  yds. 
(2  to  2.5  cbm.)  are  of  organic  origin — about  0.3  per  cent,  of  the 
total  volume  of  the  sewage.  This  small  proportion  represents 
the  really  valuable  material,  which  has  a  possible  theoretical 
value  of  from  $7.14  to  $9.52  (30  to  40  M.)  and  must  often  be 
obtained  by  elaborate  treatment.  Sometimes  the  cost  of  the 
plant  is  recovered  by  the  valuable  ingredients  found  in  the  wastes 
from  certain  industries — wool-washing  or  metal  working,  for 
instance — but  the  plant  should  then  be  used  only  for  these  valu- 
able wastes.  Sometimes  the  income  is  even  in  excess  of  the 
expense. 

The  incalculable  benefit  derived  from  a  rapid  removal,  as  well 
as  the  expense  resulting  were  any  other  method  employed,  should 
be  added  to  the  income  earned,  together  with  the  proceeds  from 
the  product  obtained.  The  same  thing  is  true  regarding  losses 
which  may  accrue  from  depreciation  of  the  neighboring  land,  or 
demands  for  damages.  These  may  amount  to  large  sums,  ac- 
cording to  the  location  of  the  plant,  and  possibly  result  in  an 
entire  change  in  the  method  of  treatment. 

In  calculating  on  a  possible  revenue  from  any  method,  care 
should  be  taken  to  estimate  the  probable  selling  price  of  the  prod- 
uct, or  the  value  of  the  ingredients  of  the  sludge  which  are  to  be 
utilized.  Above  all,  one  should  note  the  difference  between  the 
theoretical  value  as  worked  out  on  paper  and  the  actual  value, 
neglect  of  which  has  often  necessitated  the  abandonment  of  a 
process  which,  on  paper,  gave  promise  of  a  large  revenue.  The 
existence  of  an  ample  demand  for  the  product  should  also  be 
considered. 

On  account  of  the  advantages  mentioned,  which  it  is  often 
impossible  to  express  directly  in  dollars  and  cents,  but  which 
nevertheless  accrue  to  a  city  in  the  form  of  improved  hygienic 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     83 

conditions,  or  by  reducing  the  operating  charges,  private  enter- 
prises must  always  be  at  a  disadvantage,  even  when,  as  is  fre- 
quently the  case,  the  sludge  is  delivered  to  them  just  as  it  is 
obtained,  without  cost. 

The  utilization  of  sludge  may  be  accomplished  in  the  following 
ways : 

a.  By  availing  of  its  fertilizing  value. 

b.  By  availing  of  its  calorific  value  through  incineration. 

c.  By  the  gas  produced. 

d.  By  the  grease  obtained. 

e.  By  various  other  methods  of  disposal. 

Those  methods  are  comprised  under  e  which  are  of  minor  im- 
portance, or  which  seek  merely  to  render  sludge  inoffensive 
without  reference  to  its  commercial  value. 

It  must  be  emphasized  that  none  of  the  methods  as  yet  em- 
ployed entirely  fulfill  the  conditions  mentioned  at  the  beginning 
of  this  section. 

a.  UTILIZATION  OF  THE   FERTILIZING   PROPERTIES    OF    SLUDGE 

The  principal  field  for  the  use  of  settled  sludge  is  as  a  fertilizer 
in  farming  operations.  This  is  the  most  ancient  and  was,  for- 
merly, the  only  use. 

In  the  future  the  greater  part  will  also  be  utilized  in  this  way, 
especially  in  small  places  where  the  cost  of  plants  for  further 
treatment  would  be  too  great  and  where  the  small  amount  of 
sludge  produced  would  not  admit  of  its  utilization.  Other  condi- 
tions there,  too,  are  found  to  be  most  favorable  for  this  disposi- 
tion of  sludge. 

The  value  of  sludge  as  a  fertilizer  depends  chiefly  upon  the 
amount  of  nitrogen  and  phosphoric  acid  contained;  also,  in  less 
degree,  on  the  amount  of  potash.  The  first  two  each  comprise 
about  1.5  per  cent,  of  the  dried  material  in  settled  sludge,  potash 
about  0.5  per  cent.  These  figures  naturally  vary  with  sludge 
obtained  by  different  processes.  They  are,  therefore,  not  to  be 
given  equal  weight  in  their  consideration  for  agricultural  pur- 
poses. 

Detritus  from  grit  chambers  has  little  fertilizing  value  and  is 
used  principally  for  filling  in  land,  also  for  the  top  dressing  on 
irrigation  fields.  It  is,  however,  sometimes  mixed  with  the  sedi- 
ment from  tanks  to  avoid  the  expense  of  the  separate  transporta- 


84  SEWAGE  SLUDGE 

• 

tion  of  these  small  quantities.  Sludge  from  chemical  precipita- 
tion and  septic  tanks  possesses  but  little  fertilizing  value.  If  lime 
is  used  in  the  former  it  can  be  employed  where  the  soil  is  lacking 
in  this  ingredient.  Sedimentation  processes  and  bar  and  mesh 
screens  furnish  sludge  of  the  highest  value,  especially  the  last,  as 
the  detritus  from  these  is  almost  wholly  organic,  while  about  one- 
half  the  dried. material  of  settled  sludge  is  organic. 

The  fertilizing  property  of  settled  sludge  is  often  unfavorably 
affected  by  the  grease  -contained.  This  prevents  disintegration, 
to  a  great  extent,  and  injures  the  soil  by  the  formation  of  a 
coating  not  readily  pervious  to  air  or  water. 

With  greasy  sewage  a  separation  of  the  grease,  as  in  the 
Kremer  apparatus,  is  of  great  advantage  in  the  sale  of  sludge  for 
agriculture. 

The  amount  of  material  valuable  for  the  nourishment  of  plants, 
mentioned  above,  corresponds  to  a  theoretical  fertilizing  value  of 
28  cts.  per  cubic  yard  (1.50  M.  per  cbm.)  of  wet  sludge  containing 
90  per  cent,  moisture,  and  $1.10  per  cubic  yard  (6  M.  per  cbm.)  of 
dried  sludge  with  60  per  cent,  of  moisture.  With  an  amount  of 
sludge  equal  to  0.786  cu.  yd.  of  sludge  per  1000  persons  (0.6  1. 
per  capita)  daily,  or  in  round. numbers,  290  cu.  yds.  per  1000 
persons  (220  1.  per  capita)  per  year,  the  income  would  be  $78.50 
per  1000  persons  (0.33  M.  per  capita)  annually,  which  would 
cover  the  greater  part  of  the  operating  expenses — with  sedimen- 
tation processes,  under  favorable  circumstances,  the  whole. 

But  this  is  not  the  case.  The  cost  of  transportation  to  the 
place  of  utilization  should  be  deducted  from  this  theoretical 
value.  This  is  not  insignificant,  as  much  water  must  be  carried, 
even  when  the  sludge  is  quite  firm,  aside  from  the  sandy  portions 
which  are  useless  for  fertilizing. 

The  fertilizing  material  in  sludge  cannot  be  wholly  utilized, 
as  is  the  case  with  sewage  irrigation;  for  the  proportion  available 
as  plant  food  yields  an  excess  of  nitrogen.  With  grain  this 
results  in  an  abundance  of  straw,  but  few  shriveled  grains.  If, 
then,  the  nitrogen  is  to  be  entirely  utilized,  either  one  must  not 
apply  too  much  sludge,  unless  vegetables  only,  or  leafy  plants, 
are  to  be  raised,  or  else  the  lime  and  potash  which  are  lacking 
must  be  brought  to  the  field  independently. 

For  these  reasons  the  actual  value  is  much  less  than  the 
theoretical.  Moreover,  artificial  fertilizers  are  now  much  cheaper 
than  formerly  and  are  preferred,  because  more  easily  handled. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     85 

The  night  soil  from  towns  not  provided  with  sewerage  by  water 
carriage  is  superior  to  sludge  for  its  fertilizing  properties.  Sludge 
can  only  be  used  for  agricultural  purposes  in  the  fall  and  winter 
up  to  the  time  of  spring  planting,  as  the  nitrogen  is  deleterious 
to  most  plants  after  May  and,  moreover,  the  teams  for  hauling 
are  otherwise  employed.  A  constant  removal  is  desirable  for 
clarification  plants.  Only  plants  with  septic  treatment  are 
adapted  to  annual  or  semi-annual  removal. 

It  is  impossible  to  secure  high  prices,  as  farmers  are  not 
dependent  upon  sludge  for  fertilizing,  while  the  treatment  plants 
demand  constant  removal  and  the  storage  of  sludge,  except  in 
small  quantities,  is  objectionable. 

Sludge  is  utilized  as  a  fertilizer  either  wet  or  in  a  de-watered 
condition,  or  the  drying  may  be  carried  to  such  a  point  that  it 
can  be  strewn  over  the  ground. 

1.  THE  USE  OF  WET  SLUDGE  AS  A  FERTILIZER 

AY  hen  wet  the  sludge  can  either: 

1.  be  taken  to  the  fields  in  casks  or  water-tight  receptacles,  or 

2.  conveyed  thereto  in  pipes  or  open  channels. 

The  first  method  is  particularly  advisable  where,  in  small 
plants,  the  sludge  is  removed  from  the  settling  tanks  to  a  wagon 
by  suction,  or  by  pneumatic  apparatus  such  as  is  used  for  empty- 
ing cesspools.  All  bad  odors  are  thus  avoided,  and  also  the 
nuisance  of  flies.  The  sludge  treatment  plants  are,  too,  reduced 
to  a  minimum,  as  no  additional  apparatus  is  necessary.  A  pre- 
vious drawing  off  of  the  turbid  liquid  in  the  sludge  well  or 
vacuum  receiver  is  of  a  certain  advantage. 

In  larger  installations  the  sludge  container  should  be  placed 
at  such  an  elevation  that  a  wagon  can  be  filled  independently  of 
the  removal  of  the  sludge  from  the  tanks,  so  that  the  process 
may  not  be  unduly  prolonged.  For  this  purpose  a  tower  40  ft. 
(12  m.)  high  has  been  constructed  at  Mannheim,  which  holds  an 
iron  receptacle  having  a  capacity  of  15.7  cu.  yds.  (12  cbm.). 
The  Frankfort  plant  has  two  receptacles  for  sludge  removal 
standing  side-by-side  (Fig.  28) . 

It  is  possible,  also,  to  provide  pits  for  the  temporary  storage 
of  sludge,  especially  when  it  is  removed  by  the  management  of 
the  plant,  as  the  process  need  not  then  be  interrupted  in  case 
there  should  happen  to  be  no  field  prepared  for  its  reception. 


86 


SEWAGE  SLUDGE 


As  a  large  amount  of  water  must  be  carried  as  useless  ballast 
in  this  way  it  is  only  practicable  for  short  hauls  and  small  areas. 

Where  several  parties  take  the  sludge  regularly  the  second 
method  is  preferable,  and  a  system  of  piping  or  channels  should 
be  laid  for  its  distribution.  Open  channels  or  gutters  can  only 
be  employed  when  the  land  on  which  sludge  is  to  be  applied  lies 
much  lower  than  the  plant.  Particles  of  sludge  frequently 
lodge  in  the  channels  and  putrefy. 

Underground  pipe  systems  with  branches  rising  to  the  surface 
at  suitable  ooints  and  closed  with  valves  or  blank  flanges  are 


FIG.   28 — Sludge  Holders.      (Frankfort.) 

always  to  be  preferred.  Movable  lines  of  pipe  can  be  laid  on  the 
surface  from  these,  by  which  the  sludge  can  be  spread  over  the 
entire  field. 

In  Mannheim,  where  nearly  all  the  sludge  is  thus  utilized  on  a 
tract  of  land  of  about  740  acres  (300  ha.) ,  the  underground  pipes, 
which  are  about  1.25  miles  (2  km.)  long,  and  are  laid  in  ground 
owned  by  the  city,  are  of  iron  and  of  5.85  in.  (150  mm.)  inside 
diameter.  The  sludge  is  then  carried  to  private  lands  in  surface 
pipes  which  also  serve  for  a  further  distribution  on  the  city 
fields.  These  are  made  of  old  boiler  tubes  3.9  in.  (100  mm.) 
diameter  inside,  with  flanged  ends.  Where  these  are  to  remain 
for  a  long  time,  however,  they  are  furnished  with  screw  joints. 
A  loose  joint  is  sufficient  for  the  distributing  pipes  as  the  oozing 
sludge  dries  rapidly  and  closes  the  joints  sufficiently.  The  pipes 
can  be  easily  shifted  by  two  men  with  short  iron  rakes  and  drawn 
together.  (Fig.  29.) 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     87 

The  sludge  is  propelled  onward  by  means  of  a  piston  pump. 
Provision  should  be  made  here  for  forcing  water  into  the  pipes  to 
flush  them  out.  This  may  be  found  particularly  necessary  when 
branch  pipes  have  been  long  out  of  use,  so  that  the  sludge  remain- 
ing in  them  has  become  thick.  Such  obstructions  can  always  be 
removed  by  flushing.  As  the  pressure  can  always  be  increased 
at  option  with  a  piston  pump,  and  also  reduced  in  case  of  a 
break  in  the  pipes,  this  is  better  adapted  for  use  with  a  long  line 
of  pipe  than  compressed  air  from  a  receiver.  The  natural  hydro- 
static pressure  from  elevated  sludge  tanks  can  only  be  used 


FIG.  29. — Sludge  distribution  pipes.     (Mannheim.) 

where  there  are  also  means  for  increasing  this  pressure  by  con- 
necting the  distributing  pipes  directly  with  the  pump  or  with  a 
compressed  air  receiver  when  an  obstruction  occurs  in  the 
system.  In  general,  interruption  of  operation  seldom  occurs,  but 
the  use  of  such  alternate  appliances  should  be  availed  of  more 
frequently,  especially  if  the  land  suitable  for  drying  beds  lies  at 
some  distance  from  the  plant. 

If  sludge  is  to  be  removed  promptly  from  the  tanks  a  larger 
sludge  well  should  be  provided  so  that  the  machinery  for  its 
propulsion  may  be  of  smaller  size,  and  so  that  the  operation  may 
extend  over  a  longer  period. 

The  largest  plant  of  this  kind  is  at  Birmingham.  The  distance 
propelled  here  is  about  3.5  miles  (5  to  6  km.),  and  masonry 
manholes  are  built  for  connecting  the  distributing  pipes. 

Sludge  can  be  distributed  in  various  ways  on  the  fields.     A 


88  SEWAGE  SLUDGE 

method  much  in  vogue  in  England  is  to  dig  ditches  from  20  to 
36  in.  (0.5  to  0.9  m.)  wide  and  12  to  20  in.  (0.3  to  0.5  m.)  deep  at 
a  distance  of  about  5  ft.  (1.5  m.)  apart.  (Fig.  30.)  After  one  or 
two  months,  when  the  soil  is  dried  out,  these  are  filled  with 
sludge  and,  after  a  few  days,  in  order  to  prevent  objectionable 
odors,  covered  to  a  depth  of  2  to  3  in.  (5  to  8  cm.)  with  the  earth 
which  has  been  excavated  and  placed  upon  the  intermediate 
strips.  This  is  repeated  if  the  sludge  should  soak  through  the 
covering  in  wet  weather.  The  land  is  cultivated  for  one  or  two 
years,  after  which  a  new  series  of  trenches  is  excavated  on  the 
intermediate  strips  and  utilized  in  the  same  way.  A  piece  of 


FIG.  30.— Sludge  trenches. 

land  is  usually  taken  of  such  size  that  the  accumulated  sludge  of 
a  year  can  be  cared  for.  Sometimes  the  second  application  is 
made  in  connection  with  the  first.  It  is  then  wise  to  let  the  land 
lie  fallow  for  a  year  before  putting  it  under  cultivation. 

As  the  sludge  is  buried  in  thick  layers  and  is  withdrawn  from 
the  influence  of  the  sun  and  wind  by  the  covering  of  earth,  it 
dries  slowly.  Sometimes,  as  an  experiment,  it  has  been  buried 
deeper.  It  was  found  years  later,  however,  in  the  same  condition 
in  the  ground  and  without  having  decomposed. 

Foul  odors  cannot  be  avoided  with  certainty  even  with  the 
earth  covering  or  the  addition  of  lime.  Above  all,  digging  the 
ditches  is  costly.  In  Manchester,  with  wages  of  10.7  cts.  (45  M.) 
per  hour,  it  costs  6.95  cts.  per  gallon  per  foot  (0.25  M.  per  1.  per 
m.)  with  a  depth  of  15  in.  (0.45  m.). 

This  method  has  therefore  been  abandoned  in  Birmingham  in 
favor  of  that  of  placing  the  sludge  on  the  fields  without  any 
special  preparation,  as  this  plan  has  been  found  satisfactory  at 
Mannheim  and  Frankfort-on-the-Main.  Here  about  73,000 
cu.  yds.  (56,000  cbm.)  of  liquid  sludge  was  utilized  in  this  way 
during  the  year  1902.  Another  method  has  now  replaced  it, 
however,  in  connection  with  an  incineration  plant  for  rubbish 
which  has  been  installed  recently. 

The  whole  process  consists  in  irrigating  the  land  with  sludge. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    89 

(Fig.  29.)  By  making  a  suitable  choice  of  the  crop  to  be  fertilized 
sludge  may  be  disposed  of  the  year  around,  without  having  any 
large  plots  lying  idle.  In  Mannheim  sludge  is  placed  on  sugar 
beets  and  tobacco  in  the  late  spring  until  the  end  of  May,  after 
the  fields  have  been  sludged  in  the  winter  and  before  the  time  for 
planting  the  summer  grain  and  green  fodder.  By  the  end  of 
July  it  can  again  be  brought  to  the  fields  of  stubble.  In  this 
way  the  land  lies  idle  but  two  months. 

In  order  to  regulate  the  operation  it  is  necessary  that  the 
tenants,  if  the  lands  belong  to  the  city,  guarantee  to  take  the 
sludge  at  certain  times,  or  agree  among  themselves  at  what 
times  it  shall  be  taken.  It  should  be  left  to  the  farmers  to  decide 
which  fields  should  receive  the  sludge. 

About  160  cu.  yds.  (120  cbm.)  per  day  with  91  -per  cent, 
moisture  is  given  away  in  Mannheim,  while  only  47.6  cts.  (2  M.) 
is  charged  per  day  for  the  use  of  the  pipes,  besides  the  wages  of 
the  workmen  who  lay  the  pipes  and  apportion  the  sludge.  This 
returns  an  income  of  $428  (1800  M.).  An  attempt  is  also  made 
to  derive  a  profit  from  the  sludge,  as  the  123.5  acres  (50  ha.) 
of  municipal  land  has  risen  in  rental  value  from  $8.70  to  $12.50 
per  acre  (90  M.  to  130  M.  per  ha.) — about  50  per  cent.,  so  that 
quite  a  sum  is  realized  to  cover  the  cost  of  purification.  This 
shows  how  the  method  is  employed  by  the  farmers.  A  com- 
parison of  the  crops  raised  on  irrigated  and  non-irrigated  land 
also  shows  the  advantages  derived. 

The  same  favorable  results  have  been  obtained  at  Birmingham. 
Formerly  26  men  were  employed  to  bury  about  520  cu.  yds. 
(400  cbm.)  of  sludge  .daily,  while  now  only  6  men  are  needed  to 
spread  it  on  the  land.  This  is  also  shown  by  the  cost.  In  the 
first  case  the  cost,  including  pumping,  sinking  fund  and  interest 
charges  and  rent  for  the  land,  was  7  cts.  per  ton  (0.33  M.  per  long 
ton)  of  sludge,  in  Manchester  even  12.3  cts.  per  ton  (0.58  M. 
per  long  ton),  while  the  other  process  cost  but  2.5  cts.  per  ton 
(0.12  M.  per  long  ton). 

The  amount  of  land  required,  according  to  experience  in 
Mannheim,  was  about  1.9  acres  per  cubic  yard  (1  ha.  per  cbm.) 
of  average  sludge  received  per  day.  This  corresponds  to  a 
depth  of  about  1.45  in.  (3.7  cm.)  of  sludge  per  year,  or  a  require- 
ment of  fully  25  sq.  yds.  per  cubic  yard  (27  qm.  per  cbm.)  of 
sludge  per  year. 

In  England,  where  it  is  of  less  value  for  use  in  agriculture 


90  SEWAGE  SLUDGE 

than  its  cost,  an  accumulation  of  10  in.  (25  cm.)  per  year  is 
estimated  on  land  having  favorable  soil,  equivalent  to  an  area  of 
3.6  sq.  yds.  per  cubic  yard  (4  qm.  per  cbm.)  of  sludge.  With  a 
heavy  clay  soil  this  is  increased  to  10.8  sq.  yds.  per  cubic  yard 
(12  qm.  per  cbm.)  of  sludge  or  more,  and  with  a  medium  soil, 
7.2  sq.  yds.  per  cubic  yard  (8  qm.  per  cbm.)  of  sludge. 

This  small  area  of  land  is  partly  accounted  for  by  the  favorable 
climate  of  England,  particularly  in  winter. 

With  a  quantity  of  sludge  amounting  to  19.8  cu.  yds.  per 
million  gallons  (4  1.  per  cbm.)  of  sewage  and  a  daily  water  con- 
sumption of  40  gallons  (150  1.)  per  capita,  we  have  286  cu.  yds. 

(0  150x4x365 
—  =  0.219  cbm.  per  capita)  of  sludge  per 
-i-  VJv/vJ 

year,  and  therefore  with  10  in.  (25  cm.)  depth  of  sludge,  about 
1.2  sq.  yds.  (1  qm.)  per  inhabitant. 

As  the  land  is  only  spread  with  sludge  once  in  2  or  3  years  the 
above  values  should  be  doubled  or  trebled,  thus  approaching  the 
Mannheim  value,  which  was  estimated  for  a  yearly  application. 

The  estimates  of  Schiele  ("Mitteil.  d.  Kgl.  Versuchsanstalt," 
No.  11)  of  0.48  sq.  yds.  (0.4  qm.)  per  capita  seem  rather  low  in 
comparison. 

The  same  areas  are  needed  for  ditches  as  for  irrigation.  This 
method  is  used  in  England  more  for  drying  sludge  than  for 
agricultural  purposes. 

Low  embankments  are  sometimes  thrown  up  by  a  plough, 
therefore,  on  level  tracts  of  land,  forming  shallow  ponds  which 
are  filled  with  sludge  4  to  6  in.  (10  to  15  cm.)  deep,  and  this  is 
dug  under  when  dry.  If  the  land  is  used  in  this  way  several 
times  in  succession  it  becomes  more  impervious  and  drying  takes 
a  correspondingly  longer  time.  In  this  case  it  is  better  to  provide 
specially  drained  drying  places  and  to  remove  the  sludge  after 
it  has  been  dried. 

If  the  sludge  is  used  in  small  quantities  on  land  which  is 
devoted  to  agricultural  uses,  it  decomposes  entirely  and  the 
ground  is  always  ready  to  take  up  more  material.  As  the  sludge 
only  covers  the  ground  in  a  thin  layer  (see  Fig.  29)  it  dries 
rapidly  and  can  be  harrowed  under  without  delay.  The  land 
does  not  have  to  be  absolutely  level  if  care  is  taken  to  irrigate 
all  parts  by  the  aid  of  manual  labor,  and  to  see  that  the  pipes 
are  properly  laid. 

By  having  long  sludge  pipes  (see  Birmingham)  a  sufficiently 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     91 

large  area  of  land  can  generally  be  reached.  Sandy  soil  is 
especially  well  adapted  for  this,  as  the  sludge  dries  out  more 
quickly.  It  can,  however,  stand  larger  doses  of  sludge  if  it 
is  under  cultivation.  Land  which  is  subject  to  overflow  by 
streams  and  cannot  be  cultivated  may  be  advantageously  em- 
ployed in  irrigation.  (Mannheim).  Provision  for  under-drainage 
is  not  necessary. 

This  method  should  be  considered  where  there  is  septic  sludge 
or  fine  sludge  which  cannot  be  de-watered  by  pressing  or  centri- 
fuging,  and  where  drying  beds  cannot  be  provided.  The  former 
variety  of  sludge  is  not  very  useful  in  agriculture,  but  it  is 
quickly  disposed  of. 

Much  grease  is  undesirable,  but  it  disintegrates  more  readily 
and  harms  the  field  less  when  spread  in  thin  layers  than  if  in 
large  masses  and  lumps. 

2.  UTILIZATION  OF  DE-WATERED  SLUDGE  AS  FERTILIZER 

The  volume  of  sludge  is  reduced  to  one-fourth  or  more  by  de- 
watering  to  60  or  70  per  cent.,  thus  rendering  its  transportation 
for  long  distances  profitable. 

In  Frankfort-on-the-Main  sludge  dried  in  the  air  was  trans- 
ported as  far  as  5  miles  (8  km.) — in  Neustadt  O.-S.  even  6.85 
miles  (11  km.)  by  wagon,  and  in  the  latter  case  62  cts.  per  cubic 
yard  (3.40  M.  per  cbm.)  was  paid  when  no  lime  had  been  added, 
or  44  cts.  (2.40  M.)  with  lime.  In  1901,  $916.30  (3850  M.)  was 
realized  from  the  sludge. 

Conditions  are  seldom  so  favorable,  however.  The  character 
of  the  soil  in  that  vicinity  and  the  production  of  fertilizer  for 
agricultural  purposes  have  had  considerable  influence.  It  is, 
also,  often  advisable  to  give  the  sludge  away  to  promote  its  intro- 
duction and  for  experimental  purposes. 

In  most  cases  a  very  small  price  is  paid  for  dried  sludge,  which 
is  quite  out  of  proportion  to  the  cost  of  drying,  especially  when 
this  is  accomplished  by  presses  or  centrifugal  machines.  Detri- 
tus from  screens  always  brings  the  highest  price  on  account  of 
its  excellent  quality.  In  Torgau  48  cts.  (2  M.)  per  load  is  paid, 
in  Leipzig  36  cts.  (1.50  M.).  In  the  same  place  6  cts.  (0.25  M.) 
was  paid  for  tank  sludge  by  lessees  of  the  city,  by  others  12  cts. 
(0.50  M.)  which  was  reduced  by  6  cts.  (0.25  M.)  charges  for  load- 
ing. In  1908  the  gross  revenue  was  $318.68  (1339  M.)  when 


92  SEWAGE  SLUDGE 

there  was  a  great  demand,  while  the  cost  in  wages  for  removing 
the  dried  sludge  from  the  pits  was  about  $6854  (28,800  M.).  In 
Unna  septic  sludge  brings  36  cts.  (1.50  M.)  per  load,  in  Reckling- 
hausen  9  cts.  per  cubic  yard  (0.50  M.  per  cbm.).  This  price  is 
frequently  obtained,  rising  sometimes  to  18  cts.  per  cubic  yard 
(1  M.  per  cbm.). 

In  general  better  prices  can  be  obtained  in  small  places  than  in 
large  cities,  as  the  small  farmers,  who  are  the  principal  users  of 
sludge,  are  found  there  in  comparatively  greater  numbers.  Large 
plants  are  glad  to  get  rid  of  sludge  in  any  way  and  to  have  it 
called  for,  to  avoid  the  costs  of  transportation. 

In  England  they  sometimes  obtain  18  cts.  per  cubic  yard 
(1.0  M.  per  cbm.)  for  pressed  cake,  but  9  to  13  1/2  cts.  (0.50  to 
0.75  M.  per  cbm.)  is  often  paid  to  have  it  carried  away. 

The  sludge  is  spread  on  the  land  and  dug  under  as  in  the  case 
of  manure.  The  loose  consistency  of  centrifuged  sludge  renders 
it  easy  to  handle.  The  freezing  of  sludge  spread  on  the  fields, 
which  is  common  when  ponded,  promotes  its  decomposition  and 
enhances  its  fertilizing  power. 

Half  dried  sludge  can  be  frozen  in  cold  localities,  making  it 
easier  to  load  and  transport.  The  thawing  at  the  places  for  dry- 
ing, however,  entails  more  work. 

Detritus  from  screens,  possibly  also  from  grit  chambers,  is  often 
composted  with  street  sweepings,  sifted  dust,  and,  in  smaller 
quantities,  with  dried  leaves  or  pieces  of  peat,  to  increase  its 
fertilizing  qualities.  This  is  best  accomplished  in  walled  pits,  in 
which  they  are  placed  in  rotation  in  thin  layers.  The  odors  are 
less  with  the  small  surface  exposed  than  if  placed  in  heaps,  and 
when  the  pit  is  full  can  be  lessened  by  the  application  of  a  layer 
of  peat.  In  Cologne  4  pits  with  a  capacity  of  130  cu.  yds.  (100 
cbm.)  each  are  used  for  this  purpose. 

Wet  sludge  is  also  mixed  with  dust  or  street  sweepings  to 
obtain  a  material  that  is  more  easily  handled,  and  in  Cassel,  for 
example,  tanks  were  arranged  with  embankments  of  street 
sweepings,  into  which  liquid  sludge  was  pumped.  Masses  of  the 
sweepings  from  the  sides  were  then  gradually  mixed  with  the 
sludge  until  equalling  half  its  volume.  In  half  a  year  a  material 
capable  of  transportation  was  obtained  with  45  per  cent,  of 
moisture,  having  the  odor  of  garden  compost,  which  was  given 
away  free.  One  and  seven-tenths  pounds  of  lime  per  cubic 
yard  (1  kg.  per  cbm.)  was  added  to  prevent  foul  odors  and  flies, 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     93 

but  this  was  not  entirely  successful.     The  fertilizing  value  was 
also  reduced  and  the  cost  increased. 

It  is  not  advisable  to  compost  wet  tank  sludge  with  street 
sweepings,  as  this  is  only  possible  by  forming  great  heaps,  which 
delays  the  drying  and  increases  the  foul  odors.  Sweepings, 
moreover,  are  more  easily  cared  for  by  themselves. 

3.  PRODUCTION    OF   FERTILIZER    WHICH    CAN    BE    STREWN 
OVER  THE  GROUND 

Many  experiments  have  been  made  to  secure  a  fertilizer  that 
can  be  strewn  after  drying  the  sludge  to  10  or  20  per  cent, 
moisture,  such  as  has  been  obtained  in  the  form  of  poudrette 
from  fecal  matter. 

They  have  almost  all  failed  because  of  the  great  cost  required 
for  artificial  drying. 

The  process  is  made  cheaper  by  spreading  the  de-watered 
sludge  in  thin  layers  under  cover,  rolling  it  several  times  or  raking 
it  over.  In  summer  the  contained  water  can  be  reduced  to  from 
10  to  20  per  cent.  This  method  can  only  be  used  for  small 
quantities  on  account  of  the  space  required,  and  cannot  be 
employed  in  damp  weather. 

With  larger  quantities  the  de-watered  sludge  can  be  formed 
into  briquettes  and  dried  in  the  air  under  sheds,  like  bricks,  as 
described  in  the  last  section. 

In  either  case  it  can  be  ground  up,  producing  a  fertilizer  that 
can  be  strewn. 

This  method  requires  a  prolonged  handling  of  the  sludge.  The 
workmen  come  into  contact  with  it,  also,  and  it  remains  piled  up 
a  long  time,  although  it  is  not  very  offensive. 

This  process  is  considerably  accelerated  by  the  artificial  drying 
already  mentioned.  Extensive  experiments  have  been  made  in 
Frankfort  with  an  apparatus  constructed  by  Fellner  and  Ziegler, 
in  Bockenheim. 

Water  is  drawn  off  from  the  sludge,  which  has  been  placed  in  a 
rotary  drum,  filled  with  iron  balls,  at  a  temperature  of  from  212° 
to  248°  F.  (100°  to  120°  C.)  and  at  the  same  time  the  dried  mass 
is  ground.  The  fertilizer  which,  with  a  water  content  of  from 
5  to  15  per  cent,  can  then  be  strewn,  contains  1.8  per  cent,  of 
nitrogen  and  1.9  per  cent  of  phosphoric  acid.  The  whole  con- 
tains, on  an  average,  47.5  per  cent,  of  organic  matter. 


94  SEWAGE  SLUDGE 

From  0.388  cu.  yds.  (296  1.)  of  sludge  with  72  per  cent,  water 
(specific  gravity  =  1.15)  there  were  obtained  hourly  209  Ibs. 
(95  kg.)  of  poudrette,  while  101  Ibs.  (46  kg.)  of  coal  was  used. 
From  this  we  have  the  unfavorable  factor  for  the  production  of 
100X101  /  100X46  1 


The  large  consumption  of  coal  brought  the  cost  of  a  ton  of 
poudrette  to  $6.48  (3  M.  per  100  kg.)  which  was  much  higher 
than  the  price  for  which  it  could  be  sold. 

The  nuisance  produced  by  the  foul-smelling  gases  resulting 
from  the  drying  could  not  be  entirely  prevented,  even  by  con- 
ducting the  gases  under  the  fire. 

The  same  experience  was  had  in  Potsdam,  where  similar  experi- 
ments were  carried  on  in  the  further  drying  of  lignite  sludge  to 
secure  a  more  perfect  incineration. 

Nor  has  the  result  sought  been  reached  in  the  attempt  to 
secure  a  product  that  can  be  strewn  by  drying  between  hot 
cylinders,  pressed  sludge  containing  an  addition  of  lime. 

In  England,  on  the  contrary,  very  successful  methods  have 
been  devised  for  securing  a  fertilizer  that  will  find  a  market,  as 
Schiele  reports  in  No.  11  of  Mitteilung  der  Koniglichen  Versuch- 
sanstalt. 

In  Kingston  the  sludge  obtained  from  the  A.  B.  C.  process  is 
pressed  and  then  artificially  dried  and  screened.  After  further 
drying  in  the  air  during  storage  it  is  sold  as  "Native  Guano," 
especially  as  a  fertilizer  for  flowers,  at  a  price  beyond  its  true 
value.  In  spite  of  this  the  clarification  and  further  treatment 
cost  about  40  1/2  cts.  (1.70  M.)  per  capita  per  yeat.  But  the 
employment  of  a  cheaper  method  of  clarification  combined  with 
a  discharge  to  the  sea  would  be  yet  more  expensive. 

In  Glasgow  the  "Globe  Fertilizer"  is  produced  in  a  similar 
manner  by  clarifying  with  lime  and  sulphate  of  iron  and  then 
artificially  drying  the  pressed  sludge  at  a  temperature  of  149° 
to  158°  F.  (65°  to  70°  C.).  In  5  years  1.23  million  tons  (1.1 
million  long  tons)  of  sludge  with  91  per  cent,  moisture  produced 
198,000  tons  (177,000  long  tons)  of  pressed  cake  and  6400  tons 
(5700  long  tons)  of  fertilizer.  The  former  cost  42  1/2  cts  per 
ton  (2  M.  per  long  ton)  and  was  sold  for  from  14.9  to  21.2  cts.  per 
ton  (0.70  to  1.00  M.  per  long  ton);  the  latter,  while  costing 
$2.12  per  ton  (10  M.  per  long  ton)  to  produce,  sold  for  from 
$1.70  to  $2.12,  or  in  sacks  $2.97  per  ton  (8  to  10  M.  and  14  M. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE    95 

per  long  ton).  This  return  reduced  the  cost  of  clarification  by 
81  cts.  per  million  gallons  (0.9  M.  per  1000  cbm.)  of  sewage.  In 
spite  of  the  smaller  price  the  sale  of  the  pressed  cakes  was  pre- 
ferred, as  they  required  less  work. 

No  great  profit  can  be  looked  for  by  further  treatment  of  the 
fertilizer,  especially  as  the  valuable  artificial  fertilizers  at  their 
cheap  prices  are  always  preferred.  Even  with  artificial  drying 
it  is  a  long  drawn  out  process,  requiring  troublesome  manipula- 
tion. It  is  always  to  the  interest  of  the  plant  to  render  the 
process  as  short  and  simple  as  possible,  especially  where  no 
great  profit  can  be  realized. 

b.  COMPLETE  UTILIZATION  OF  CALORIFIC  VALUE  BY  BURNING 

A  means  frequently  employed  to  remove  valueless  refuse 
which,  when  accumulated,  becomes  offensive  from  its  gradual 
decomposition  and,  also,  takes  up  too  much  room,  is  by  burning. 
By  using  the  proper  apparatus  the  heat  from  the  gas  generated 
can  be  utilized,  or  if  this  is  not  done  the  volume  is  at  least  reduced 
and  the  final  product  is  an  unobjectionable  heap  of  ashes. 

The  possibility  of  burning,  and  especially  of  utilizing,  the 
calorific  value  of  sludge  depends  on  several  conditions  which  are 
of  great  importance,  the  disregard  of  which  has  led  to  many 
failures.  We  will  therefore  consider  them  somewhat  more  in 
detail. 

The  degree  to  which  sludge  can  be  disposed  of  by  making  use 
of  its  calorific  value  depends  upon  the  proportion  of  organic 
matter  contained,  on  the  water  and  on  the  grease. 

The  effective  thermal  value  of  any  material  is  the  number  of 
heat  units  developed  by  the  combustion  of  one  pound  (or  of  1  kg.) 
of  the  fuel  under  consideration. 

The  larger  the  proportion  of  organic  constituents  in  sludge  the 
greater  the  amount  of  available  heat  which  is  released  by  com- 
bustion. The  inorganic  material  and  the  water  contained  in  it 
absorb  a  certain  part  of  the  heat  generated  in  raising  their  own 
temperature.  Water  consumes  a  large  part  of  it,  not  only 
because  it  constitutes  a  great  proportion  of  the  whole,  but  par- 
ticularly because  it  must  be  converted  into  steam.  180  b.  t.  u. 
per  pound  are  required  to  raise  water  from  32°  F.  to  212°  F.  (100 
calories  per  kg.  from  0°  to  100°  C.),  but  to  convert  water  at  212° 
F.  (100°  C.)  into  steam  of  the  same  temperature  there  are  required 


96 


SEWAGE  SLUDGE 


965  b.  t.  u.  per  pound  (536  calories  per  kg.),  altogether  about 
1152  b.  t.  u.  per  pound  (640  calories  per  kg.). 

As  in  ordinary  incineration  plants  only  the  amount  of  heat  can 
be  utilized  comprised  between  the  temperature  of  combustion 
and  that  of  the  gases  discharged,  and  as  this  last  temperature  is 
usually  above  212°  F.  (100°  C.),  the  water  contained  in  the  fuel  is, 
as  a  rule,  converted  into  steam.  The  heat  used  in  this  process  is, 
however,  lost,  except  for  a  part  which  appears  in  the  higher 
temperature  of  the  gases  given  off. 

The  effect  of  the  moisture  on  the  burning  can  be  shown  mathe- 
matically. 

The  absolute  caloric  value  of  the  dried  material  is  about  7200 
b.  t.  u.  per  pound  (4000  calories  per  kg.) .  With  a  water  content 
of  90  per  cent,  there  will  be  90  Ibs.  (or  kg.)  of  water  in  100  Ibs. 
(or  kg.)  of  sludge.  To  convert  this  into  steam  requires  90  X  1152 
=  103,680  b.  t.  u.  (90X640  =  57,600  calories).  The  calorific 
value  of  the  dried  material  is  10x7200  =  72,000  b.  t.  u.  (10x4000 
=  40,000  calories). 

Sludge  containing  so  much  water  has  therefore  no  practical 
heating  value.  It  is  only  when  it  is  further  dried  that  we  obtain 
from  the  calorific  value  of  the  material  an  excess  of  heat  above 
that  used  up  in  the  generation  of  steam.  The  following  table  by 
Koschmieder  (Tech.  Gemeindeblatt,  seventh  year)  shows  to 
what  degree  this  occurs  through  the  reduction  of  the  contained 
moisture  by  10  per  cent. 


With  10  Ibs.  of  dried'  material 

With  10  kg.  of  dried  material 

B.t.u.  =  10X7200  =  72,000 

Calories  =  10  X  4000  =  40,000 

Per  cent, 
moisture 

Water 
Ibs. 

To  convert  to 
steam  requires 
b.t.u.  (about) 

Resulting  ex- 
cess of  heat 
b.t.u.  (about) 

Water 
kg. 

To  convert  to 
steam  requires 
calories  (about) 

Resulting  ex- 
cess of  heat 
in  calories 

90 

40 

103,680 

-31,680 

!   90 

57,600 

-  17,600 

80 

40 

46,080 

+  25,920 

40 

25,600 

+  14,400 

70 

23.3 

26,842 

+  45,158 

23.3 

14,912 

+  25,088 

60 

15 

17,280 

+  54,720 

1    15 

9,600 

+  30,400 

50 

10 

11,520 

+  60,480 

10 

6,400 

+  33,606 

40 

6.7 

7,718 

+  64,282 

6.7 

4,288 

+  35,712 

30 

4.3 

4,954 

+  67,046 

4.3 

2,752 

+  37,248 

20 

2.5 

2,880 

+  69,120 

2.5 

1,600 

+  38,400 

10 

1.1 

1,267 

+  70,733 

1.1 

'704 

+  39,296 

TREATMENT  AND  UTILIZATION  OF  SLUDGE     97 

Sludge  has  a  practical  calorific  value  only  after  the  contained 
moisture  has  been  reduced  ,to  80  per  cent.  This  value  increases 
gradually  in  proportion  to  the  decreasing  amount  of  water  to  be 
driven  off  at  decreasing  intervals  of  10  per  cent.  (See  Chapter 
IV.)  It  appears,  therefore,  that  little  will  be  gained  by  drying  it 
beyond  50  or  60  per  cent. 

It  should  also  be  noted  that  the  temperature  must  not  be 
lowered  below  the  degree  required  for  combustion  when  material 
to  be  burned  is  added. 

The  grease  contained  is  a  disadvantage  in  burning  inasmuch  as 
this  must  be  distilled  off  at  a  temperature  of  572°  F.  (300°  C.) 
before  reaching  the  temperature  of  combustion.  The  heat  thus 
used  is  lost,  as  these  distilled  vapors  are  not  consumed  in  any 
ordinary  furnace.  With  every  addition  of  material  to  be  burned, 
therefore,  the  moisture  must  be  turned  into  steam,  the  grease  and 
similar  substances  must  be  distilled  off,  and  it  is  only  with  a  yet 
higher  temperature  that  we  obtain  gases  of  the  degree  of  heat 
required  for  practical  use. 

When  burning  dried  sludge  in  house  stoves  the  grease  is  dis- 
tilled off  and  is  quickly  deposited  on  the  cooler  parts  of  the  heat- 
ing apparatus,  resulting  in  much  dirt  and  the  production  of  im- 
pure gases  which  escape  into  the  room  and  render  the  air  impure. 
When  it  is  burned  under  steam  boilers  the  gases  escaping  through 
the  chimney  annoy  the  people  of  the  neighborhood. 

Although  we  find  from  the  above  that  sludge  sufficiently  dried 
can  be  burned  without  adding  other  material,  its  use  is  too  limited 
to  make  further  drying,  except  by  the  usual  methods,  of  economic 
value.  Experiments  in  this  direction  have,  however,  been  made. 

In  Frankfort-on-the-Main  briquettes  made  from  de-watered 
sludge  with  brick  presses  and  dried  in  the  air  to  10  per  cent,  of 
moisture  gave  an  effective  calorific  value  of  9910  b.  t.  u.  (2500 
calories),  on  an  average  with  47  per  cent,  of  combustible  material. 

According  to  Bredtschneider  and  Proskauer  (Vierteljahres- 
schrift  f.  off.  Ges.— Pflege.,  Vol.  XXXVII)  air-dried  sludge  with 
20  per  cent,  moisture  contains  about  30  per  cent,  combustible 
material  and  about  50  per  cent,  mineral  matter,  and  has  a 
calorific  value  of  8730  b.  t.  u.  (2200  calories). 

Experiments  at  Elberfeld  have  shown  that  sludge  with  as 
much  as  60  per  cent,  moisture  can  be  burned  without  the  addition 
of  coal  by  using  a  forced  draft. 

As  valuable  combustible  materials,  such  as  carbonic  acid  and 
7 


98  SEWAGE  SLUDGE 

methane,  are  lost  during  decomposition,  sludge  from  septic  tanks 
is  less  suitable  for  heating  purposes.  In  Stuttgart  the  calorific 
value  of  septic  sludge  with  40  per  cent,  moisture  was  6456  b.  t.  u. 
(1627  calories) ;  with  settled  sludge  containing  47  per  cent,  mois- 
ture, 8035  b.  t.  u.  (2025  calories).  In  comparison  we  have: 
lignite  sludge  from  Potsdam  with  60  per  cent,  moisture,  5950 
b.  t.  u.  (1500  calories),  and  settled  sludge  from  Hanover,  dried  at 
212°  F.  (100°  C.),  15,870  b.  t.  u.  (4000  calories),  with  28  per  cent, 
ash  and  17,120  b.  t.  u.  (4315  calories) ,  with  18.5  per  cent.  ash.  A 
certain  increase  in  calorific  value  is  found  in  coal  mining  districts 
due  to  particles  of  coal  in  the  sewage. 

Coal  or  other  combustible  material,  such  as  dust,  has  been 
mixed  with  sludge  to  increase  its  calorific  value  in  order  to  render 
its  use  practicable.  The  coal  can  be  added  to  the  sewage,  to  the 
wet  sludge  or  to  the  dried  sludge  before  it  is  used. 

The  first  method  brings  out  the  most  economical  use  of  the 
material  added.  It  can  be  used  as  a  means  for  clarification,  as 
in  the  lignite  process.  The  addition  of  coal  or  peat  also  facilitates 
drying  in  the  air  as  well  as  by  pressure,  and  retards  the  decom- 
position of  deposited  sludge.  If  the  material  that  is  to  be  burned 
is  mixed  with  wet  sludge,  this  is  best  in  the  form  of  coal  dust, 
which  is  thoroughly  incorporated  with  the  sludge  by  a  process 
patented  by  the  firm  of  Rothe,  in  which  it  is  sucked  in  like  por- 
ridge through  a  lateral  suction  pipe  in  the  sludge  pump. 

The  complete  utilization  of  the  calorific  value  is,  therefore,  the 
approved  practice  in  using  the  lignite  process,  and  as  Reichle  and 
Dost  have  shown  by  experiments  to  be  described  later,  the  foul 
material  contributes  from  11  to  30  per  cent,  of  the  resulting 
calorific  value. 

Experiments  at  Charlottenburg,  where  sludge  containing 
40  per  cent,  moisture  was  burned  with  the  addition  of  coal, 
more  coal  was  required  to  obtain  a  certain  amount  of  steam  than 
if  coal  alone  had  been  used.  For  other  reasons,  too,  stress  should 
be  laid  upon  mixing  as  thoroughly  as  possible. 

In  the  lignite  process,  where  4.1  to  8.3  tons  per  million  gallons 
(1  to  2  kg.  per  cbm.)  of  ground  lignite  with  alum  and  sulphate 
of  iron  are  added  to  the  sewage,  the  sludge  is  usually  burned  under 
the  boilers  of  the  apparatus  just  as  it  comes  from  the  press.  If 
it  is  received  in  the  consistency  of  gruel,  which  cannot  always  be 
avoided  with  its  variable  character,  other  sludge  which  has 
received  additional  drying  in  the  air  under  cover  is  mixed  with  it. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE     99 

The  grate  is  supplied  with  diagonal  bars.  The  resulting  ash  is 
about  1  /  7  of  the  original  amount. 

In  Copenick  the  lignite  sludge  which  has  been  dried  in  tanks 
three  or  four  weeks,  is  heaped  up  under  cover  for  about  four 
months,  and  this  high  piling  up  has  been  found  of  greater  advan- 
tage than  placing  it  in  thin  layers,  so  that  the  large  drying  sheds 
have  been  but  partially  used.  The  calorific  value  with  about 
40  per  cent,  moisture  is  8630  b.  t.  u.  (2175  calories).  It  is  burned 
in  the  neighboring  electric  plant  with  an  addition  of  one  part  of 
coal  to  2  to  4  parts  of  sludge,  applying,  at  the  same  time,  a  forced 
draft.  Lignite  sludge  is  also  used  at  Potsdam  in  the  municipal 
electric  plant,  but  without  previous  storage,  using  a  mixture  of 
1  part  coal  slack  to  8  parts  sludge.  The  electric  plant,  which 
uses  no  other  fuel,  pays  therefor  $2142  (9000  M.),  which  repre- 
sents only  a  part  of  the  cost  of  the  sludge. 

An  experimental  plant  of  the  Nuremberg  Machine  Manufactur- 
ing Co.,  Inc.,  dries  the  wet  lignite  sludge  by  heat,  the  final  product 
— a  firm  black  mass — is  used  for  fuel  and  the  steam  generated 
thereby  employed  to  operate  the  machinery.  Further  particu- 
lars as  to  the  success  of  this  plant  cannot  yet  be  given. 

At  Spandau  the  lignite  sludge  is  made  into  briquettes  and 
sold  at  $1.52  per  ton  -(0.70  M.  per  100  kg.)  for  fuel.  In  other 
places  the  sludge  briquettes  which  are  not  needed  to  operate  the 
clarification  plant  are  given  away  to  poor  families. 

Experiments  with  septic  sludge  from  Emscher  tanks  show  that 
it  burns  when  containing  46.4  per  cent,  moisture  with  an  addition 
of  from  20  to  5  per  cent,  of  coal,  when  50  per  cent,  which  has  no 
commercial  use  is  left  as  ash. 

Besides  the  complete  utilization  of  the  calorific  value,  an 
attempt  should  be  made  to  secure  an  ash  having  value,  on  account 
of  its  large  amount.  This  is  especially  worthy  of  attention  where 
there  is  a  lack  of  sand  and  gravel  for  mortar  and  concrete.  At 
Huddersfield,  England,  the  pressed  sludge  from  chemical  pre- 
cipitation is  mixed  with  coke  breeze  in  the  proportion  of  5:1  and 
burned.  The  resulting  slag  is  used  with  lime  and  cement  to 
make  mortar.  The  operation  is  expensive,  however,  the  burning 
alone  costing,  in  the  added  coke  and  wages,  55.2  cts.  per  ton 
(2.60  M.  per  long  ton),  to  which  should  be  added  53.1  cts.  per  ton 
(2.50  M.  per  long  ton)  for  pressing.  The  only  advantage  lies  in 
the  fact  that  the  entire  product  is  thus  utilized  without  any  waste. 

This  point  is  not  worth  considering  in  lignite  clarification  plants 


100  SEWAGE  SLUDGE 

where  the  residue  of  ash  is  but  1/7  of  the  whole  amount.  The 
complete  utilization  of  the  calorific  value  is  of  importance  in 
reducing  the  high  cost  of  the  process,  especially  as  large  machines 
are  necessary  to  do  the  work. 

It  has  even  been  proposed  to  lessen  the  burden  on  the  sewage 
at  Berlin  by  clarifying  a  part  of  the  sewage  with  coal  and  then 
utilizing  the  sludge  in  the  production  of  electrical  energy  for  the 
railways.  With  an  addition  of  from  20  to  30  per  cent,  of  lignite 
there  has  been  estimated  a  return  of  21,431,000  k.  w.  h.  from 
164,640  tons  (147,000  long  tons)  of  sludge. 

Centrifuged  sludge  is  particularly  well  adapted  to  burning  on 
account  of  its  loose  consistency.  A  separate  extraction  of  grease 
from  the  sewage  in  order  to  produce  a  sludge  without  this  in- 
gredient is  of  advantage,  as  has  been  already  explained  in  the 
remarks  on  combustion. 

In  Bradford,  where  all  the  sludge  is  worked  over  for  the  grease, 
the  pressed  cakes  without  grease  are  mixed  with  coal  in  the 
proportion  of  7:1,  while  a  heated  forced  draft  is  used  to  prevent 
any  annoyance  from  smoke,  and  the  cakes  are  then  burned  under 
steam  boilers,  thus  saving  an  annual  expense  of  $4760  (20,000 
M.)  for  fuel. 

Briquettes  made  from  sludge  dried  to  75  per  cent.,  with  coal 
and  bitumen  added,  have  been  used  for  fuel  in  Columbus,  U.  S., 
but  as  in  all  experiments  with  a  mixture  of  more  valuable  fuel, 
the  cost  has  been  greater  than  its  actual  value. 

The  incineration  of  sludge  with  dust  or  street  sweepings  com- 
bined, has  frequently  been  found  practicable,  the  most  notable 
example  being  at  Frankfort-on-the-Main,  where  centrifuge- 
dried  sludge  has  been  treated  in  this  way.  Experiments  there 
indicated  that  sludge  with  75  per  cent,  moisture  mixed  with 
house  sweepings  would  burn  without  any  additional  fuel;  so, 
also,  in  Charlottenburg  where,  with  a  mixture  of  1  part  sludge 
with  75  per  cent,  moisture  and  3  parts  house  sweepings,  1  Ib. 
evaporated  from  0.76  to  1.08  Ibs.  (1  kg.  evaporated  0.76  to 
1.08  kg.)  of  water  in  the  boiler. 

This  method  is  frequently  used  in  England  where  pressed 
cakes  containing  50  to  60  per  cent,  moisture  are  customarily 
mixed  with  dust  in  the  proportion  of  1  to  2.  The  calorific 
value  of  the  material  is  utilized  to  operate  the  plant.  This  is, 
naturally,  not  great.  In  Bury  67  to  78  tons  (60  to  70  long  tons) 
of  the  mixed  material  are  burned  daily,  furnishing  38  h.  p.  for 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  101 

the  engine  boilers  of  the  plant.  The  slag  resulting  from  this 
method  can  be  melted  and  ground.  It  is  then  used  for  building 
material.  Especial  attention  is  given  to  the  gases  produced,  as 
these  may  easily  cause  annoyance. 

c.  PRODUCTION  OF  GAS 

The  unfavorable  results  obtained  in  utilizing  the  combustible 
material  in  sludge  as  fuel  which  have  been  described  have  ledjtct 
experiments  by  which  its  calorific  value  may  be  more  fully 
realized  through  its  conversion  into  gas. 

A  distinction  should  be  made  here  between  the  removal 
(entgasung)  of  the  gas  and  the  production  (vergasung)  of  the 
gas  from  the  sludge. 

The  former  is  accomplished  by  driving  out  the  volatile  in- 
gredients, especially  the  carburetted  hydrogen,  which  makes  a 
very  valuable  gas,  by  a  high  temperature.  This  is  dry  distilla- 
tion, such  as  is  employed  in  making  illuminating  gas. 

B^lhe.  production  of  gas  is  understood  the  combination  of  the 
carbon  contained  in  the  material  with  the  oxygen  of  the  air  to 
form  carbonic  oxide.  This  can  be  brought  about  by  passing  air 
through  glowing  coals.  Carbonic  oxide  gas  is  produced  which, 
when  mixed  with  the  inert  nitrogen  of  the  air,  is  called  producer 
gas.  This  is  not  a  perfectly  combustible  product,  as  the  carbon 
is  consumed  only  to  the  degree  necessary  to  produce  carbonic 
oxide,  but  not  carbonic  acid. 

If  steam  from  water  is  brought  into  contact  with  glowing 
coals  the  oxygen  in  it  unites  with  the  carbon,  forming  carbonic 
oxide,  and  the  hydrogen  is  set  free.  The  mixture  of  these  two 
gases  is  called  water  gas.  In  the  suction  gas  producer,  air  and 
steam  are  admitted  at  the  same  time  and  the  resulting  gas  is  a 
mixture  of  both  kinds.  These  gases  are  usually  mixed  with  other 
volatile  substances  which  are  freed  from  the  combustible  material 
recently  added  to  the  upper  layer  in  the  generator. 

At  first  attempts  were  made  to  drive  off  the  gas  from  sludge, 
but  so  far  this  has  been  without  practical  success.  The  reason 
for  failure  is  the  great  volume  of  the  water  contained  in  the 
sludge  as  compared  with  the  small  amount  of  combustible 
material,  a  condition  which  constitutes  the  great  difficulty  in 
all  methods  of  utilization.  A  great  deal  of  fuel  is  required  to 
heat  this  great  mass,  while  the  value  of  the  gases  obtained  is  not 


102  SEWAGE  SLUDGE 

in  proportion.  Moreover,  a  considerable  amount  of  residue 
remains  after  the  gas  has  been  removed. 

This  is  also  shown  in  the  experiments  undertaken  in  Tegel  to 
remove  the  gas  from  sludge  produced  by  mixing  peat  with  the 
sewage  of  Stuttgart.  Here  from  1.96  cu.  yds.  (1.5  cbm.)  of 
sludge  weighing  1700  Ibs.  (770  kg.)  there  remained  a  residue  in  the 
retort  of  534  Ibs.  (242  kg.)  =31.4  per  cent. — quite  a  large  amount 
— and  the  gas  obtained  was  about  7060  cu.  ft.  (200  cbm.)  or 
417  cu.  ft.  from  100  Ibs.  (26  cbm.  from  100  kg.)  of  sludge,  using 
1700  Ibs.  (770  kg.)  of  coke.  The  calorific  value  of  the  gases 
obtained  was  16,578  b.t.u.  (4178  calories)  as  compared  with 
possibly  19,840  b.t.u.  (5000  calories)  in  the  case  of  illuminating 
gas,  and  the  cost  of  production  (fuel  and  wages)  $1.27  per  1000 
cu.  ft.  (0.19  M.  per  cbm.). 

The  cost  of  production  is  therefore  very  high  in  proportion  to 
its  value.  The  conditions  are  still  less  favorable  with  ordinary 
settled  sludge,  as' was  shown  by  experiments  at  Frankfort-on-the- 
Main.  Here,  from  220  Ibs.  (100  kg.)  of  air-dried  briquettes  only 
690  cu.  ft.  (19.5  cbm.)  of  gas  was  obtained.  After  a  4-hour 
period  50  per  cent,  residue  was  found.  The  gas  contained: 

Hydrogen,  36  per  cent. 

Carbon,  23  per  cent. 

Methane,  13  per  cent. 

Heavy  carburetted  hydrogen,  6  per  cent. 

Carbonic  acid,  16  per  cent. 

Nitrogen,  6  per  cent. 

100  per  cent. 

Its  calorific  value  was  15,000  b.  t.  u.  (3800  calories)— the 
maximum  16,860  b.  t.  u.  (4250  calories).  The  illuminating  power 
was  small,  5.3  candle  power  while  using  5.3  cu.  ft.  (150  1.)  per 
hour,  so  that  the  gas  cannot  be  used  for  lighting. 

Similar  results  were  obtained  with  sludge  from  septic  tanks 
and  plain  sedimentation  in  Stuttgart,  as  the  following  figures 
indicate. 

There  is,  therefore,  no  question  of  economy  in  the  process. 

Experiments  for  converting  sludge  into  gas  have  resulted  more 
favorably  although,  even  here,  there  has  been  no  satisfactory 
solution. 

The  same  unfavorable  characteristics  of  settled  sludge  are 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  103 


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104  SEWAGE  SLUDGE 

shown  here  as  were  found  in  connection  with  burning,  as  might  be 
expected,  for  both  processes  aim  to  secure  its  entire  calorific 
value. 

Gas  obtained  is  used  to  drive  engines.  As  the  calorific  value 
of  the  sludge  is  small  these  engines  must  be  made  very  large. 
Besides,  the  preliminary  distillation  of  the  grease,  which  quickly 
condenses,  makes  the  engine  and  plant  for  the  production  of 
gas  very  dirty,  so  that  ample  reserve  must  be  provided  for  unin- 
terrupted power. 

The  heat  required  to  convert  the  water  into  steam  and  distill 
the  grease  might  be  recovered  by  passing  their  vapors  over  glow- 
ing coals,  possibly  by  admitting  them  into  the  fire  box,  thereby 
inducing  further  decomposition.  In  using  wet  sludge  for  making 
water  gas  the  method  suggested  by  Koschmieder  may  be  em- 
ployed: of  conveying  the  steam  from  the  sludge  to  the  generator, 
thus  utilizing  the  heat  devoted  to  that  purpose.  The  heat  from 
the  gases  may  be  used  to  further  dry  the  de-watered  sludge. 
Coal  to  the  amount  of  1  1/2  times  the  wet  sludge  should  then  be 
added.  This  increases  the  cost  of  the  process,  and  probably  in  a 
greater  degree  than  by  special  drying  of  the  sludge  or  by  other 
processes  where  the  storage  or  prolonged  treatment  of  the  sludge 
is  avoided. 

Sludge  which  has  not  had  coal  added  during  or  after  clarifica- 
tion is  not  suited  to  the  removal  of  gas. 

The  experiments  which  have  been  made  in  this  direction  by  the 
Deutz  Gas  Engine  Works  have  therefore  only  dealt  with  coal- 
clarified  sludge.  Of  these  experiments,  some  of  which  were 
undertaken  at  Deutz,  some  at  the  Dresden  Municipal  Exhibition 
and  some  at  the  experimental  plant  at  Oberschoneweide,  we  will 
only  consider  those  made  by  Reichle  and  Dost  at  the  last-named 
place,  as  the  results  are  all  similar. 

The  experimental  plant,  which  has  now  been  abandoned, 
consisted  of  a  generator,  a  condensing  plant,  and  a  suction  gas 
engine  of  70  h.  p.  With  an  addition  of  from  5.63  to  8.23  tons  of 
lignite  and  from  0.75  to  1.13  tons  of  sulphate  of  alumina  to 
1  million  gallons  of  sewage  there  were  obtained  12.5  tons  of 
sludge  (with  1.35  to  1.97  kg.  lignite  and  0.18  to  0.27  kg.  sulphate 
of  alumina  per  cbm.,  3  kg.  of  sludge)  with  64  per  cent,  moisture 
having  a  calorific  value  of  3202  b.  t.  u.  per  pound  (1779  calories 
per  kg.).  The  sludge  received  from  the  press  with  this  amount 
of  moisture  was  dried  in  the  air  to  51  per  cent.,  for,  as  was  shown 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  105 

by  these  experiments,  sludge  with  58  per  cent,  or  more  cannot 
be  converted  into  gas. 

The  following  composition  was  the  result  of  an  analysis: 

Carbon,  22.3  per  cent. 

Hydrogen,  2.7  per  cent. 

Nitrogen,  1.0  per  cent. 

Oxygen,  12.8  per  cent. 

Sulphur,  0.5  per  cent. 

Ash,  9.8  per  cent. 

Water,  5.1  per  cent. 

The  calorific  value  of  the  gases  produced  was  on  an  average 
81,000  (maximum  90,000)  b.  t.  u.  per.  1000  cu.  ft.  of  gas,  as  com- 
pared with  146,000  or  more  with  other  power  gas  [721  (maximum 
801)  calories  per  cbm.  as  compared  with  1300  or  more].  The 
engine,  therefore,  only  furnished  57  h.  p.  .  The  amount  of  lignite 
sludge  was  5.56  Ib.  (2.52  kg.)  per  brake  horse-power-hour, 

,   .  ,2510X1007 

whence  we  have  a  thermal  efficiency  of  ( 


=  14.1  per  cent.  3202  (1779)  here  represents  the  calorific  value 
of  the  sludge  and  2510  (632)  the  heat  units  required  for  one  h.  p. 
hour. 

The  impure  matter  increased  the  heating  value  by  from  11  to 
30  per  cent.,  the  sewage  being  composed  mostly  of  factory  wastes 
without  combustible  ingredients. 

As  the  cost  of  fuel  is  0.36  to  0.50  cts.  (1.5  to  2.0  pfg.)  per  h.  p. 
hour,  and  about  15  per  cent,  of  the  power  derived  from  the  sludge 
was  utilized  for  the  clarification  plant,  the  cost  of  operation  at 
Oberschoneweide,  as  was  demonstrated  in  1905,  was  reduced 
from  33.1  cts.  to  24.5  cts.  (1.39  M.  to  1.03  M.)  per  capita  per 
annum,  or  by  $16.20  per  million  gallons  (1.8  pfg.  per  cbm.)  of 
sewage,  including  interest  and  amortization  charges. 

Such  utilization  of  lignite  sludge  has  a  certain  economic 
advantage  which  can  be  increased  by  improvements  as  already 
indicated.  The  costs  of  maintenance  and  operation  may  be 
reduced  if  the  process  is  used  in  connection  with  a  municipal 
power  plant  where  sludge  is  available.  It  does  not  seem  wise  to 
adopt  the  lignite  process  merely  for  the  profitable  use  of  the 
sludge  when  its  advantages  —  freedom  from  odor,  small  area 
required,  easy  handling  and  therefore  the  possibility  of  estab- 
lishing the  plant  in  populous  districts  —  cannot  all  be  realized. 


106  SEWAGE  SLUDGE 

Recently  a  plant  of  this  kind,  to  utilize  sludge  for  power  gas, 
has  been  installed  at  Elbing,  but  although  no  decided  opinion 
can  be  expressed  after  the  short  time  it  has  been  in  operation,  it 
does  not  appear  to  meet  the  expectations  as  to  cost. 

d.  EXTRACTION  OF  GREASE. 

The  grease  contained  in  sludge  is  detrimental  to  the  methods 
of  utilization  hitherto  described.  In  order  to  realize  the  greatest 
possible  value  either  as  a  fertilizer  or  for  fuel  it  is  necessary  to 
eliminate  the  grease,  or  at  least  to  have  as  little  of  it  as  possible. 
But  the  contained  grease  itself  represents  a  valuable  product. 
It  is  necessary,  however,  to  extract  it  from  the  sludge  in  the 
simplest  and  most  economical  way. 

This  can  be  done: 

1.  By  extraction  of  the  grease  from  the  settled  sludge. 

2.  By  extraction  of  the  grease  from  the  sewage  either  by 
itself  or  in  connection  with  other  materials. 

A  mechanical  separation  of  the  grease  by  its  rising  to  the 
surface  on  account  of  its  light  specific  gravity  does  not  take  place 
in  connection  with  sludge  as  it  is  too  intimately  mixed  with  the 
particles  of  the  latter.  In  de-watering  by  centrifugal  force  in 
closed  drums  the  grease  separates  from  the  water  in  the  center 
and  can  be  collected. 

Both  of  these  methods  have  been  tried  out  in  practice. 

The  first  is  more  generally  known  in  Germany  from  a  plant 
at  Cassel  erected  by  Degener  through  the  Cassel  Machine  Works 
—Incorporated,  formerly  the  firm  of  Beck  and  Henkel. 

The  sludge,  which  was  given  away  by  the  city  of  Cassel  just  as 
it  was  obtained,  and  which  contained  coarse  sedimentary  and 
floating  matter  on  account  of  the  lack  of  grit  chambers  or  screens, 
was  worked  over  by  the  following  processes: 

1.  Freeing  the  sludge  from  its  coarse  ingredients  (rags,  sticks 
of  wood,  etc.)  by  a  rolling  screen. 

2.  Mixing  with  sulphuric  acid  in  kettles. 

3.  Heating  the  mixture  in  Montejus  [3  to  each  3.9  cu.yds. 
(3  cbm.)]  to  212°  F.  (100°  C.). 

4.  Pressing  the  heated  material  in  filter  presses.     The  drainage 
water  was  conveyed  to  a  lime  well  to  neutralize  the  acids  and 
returned  to  the  plant. 

5.  Drying  the  pressed  cakes,  first  by  introducing  steam  into 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  107 

the  presses  and  thence,  after  they  were  disintegrated,  into  an 
apparatus  heated  by  steam. 

6.  Extracting  the  grease  by  benzine  in  an  extractor  holding 
8.5  cu.  yds.  (6.5  cbm.). 

7.  Liberating  the  grease  and  sediment  from  the  benzine  by 
steam.     The  latter  was  recovered  by  condensation  and  could  be 
used  again  several  times. 

8.  Subsequent  drying  of  the  sediment  in  thin  layers  in  the  air 
or  in  a  drying  apparatus  after  the  moisture  had  been  reduced  to 
40  or  50  per  cent,  by  steam. 

9.  Distillation  of  the  grease.     This  produced  two  layers  of 
grease,  yellow  and  brown,  while  a  tar-like  material  remained  as 
residue. 

The  amount  of  grease  extracted  varied  from  8  to  25  per  cent, 
and  averaged  15  per  cent,  of  the  dried  material  in  the  sludge 
cakes,  while  the  average  amount  of  grease  contained  in  the  sludge 
was  18  per  cent. 

The  residue,  deprived  of  grease,  found  a  good  market  as  a 
fertilizer  and  had  the  following  composition  referred  to  the  dried 
material : 

Nitrogen,  2 . 35  to  5 . 90  per  cent. 

Grease,  0 . 7 1  to  5 . 89  per  cent. 

Phosphoric  acid,  0.41  to  1.12  per  cent. 

Potash,  0 . 03  to  0 . 15  per  cent. 

As  shown  above,  this  method  was  very  troublesome.  In 
addition  to  a  good  deal  of  machinery  it  required  16  men  for 
operation — 8  by  day  and  8  by  night. 

The  final  product  was  unobjectionable  from  a  sanitary  point  of 
view  and  the  process  was,  moreover,  harmless  for  the  workmen. 

As  many  alterations  and  improvements  were  found  necessary 
in  trying  out  the  plant,  the  capital  required  had  to  be  increased, 
making,  with  the  high  cost  of  operation,  economical  results 
impossible.  After  3  years,  therefore,  it  was  abandoned,  and  the 
contract  with  the  city  of  Cassel  was  cancelled. 

The  principal  reason  for  the  high  cost  of  operation  was  the 
great  expense  that  had  to  be  incurred  for  fuel  used  in  heating  the 
wet  sludge  and  in  drying  the  pressed  cakes,  which  was  out  of 
proportion  to  the  value  of  the  grease  contained  (See  page  10). 

Experiments  made  at  Frankfort-on-the-Main  to  extract  grease 
with  benzine  from  wet  sludge  containing  15  to  20  per  cent,  of 


108  SEWAGE  SLUDGE 

grease  in  the  dried  material  showed  the  process  to  be  uneco- 
nomical. 

Extracting  grease  from  sewage  sludge  obtained  under  normal 
conditions  can  never  be  profitable. 

The  situation  is  different  in  towns  where  much  grease  is  dis- 
charged into  the  sewage  from  factories,  as,  for  example,  in  various 
English  cities  from  wool  washing  works.  * 

The  largest  plant  of  this  kind  is  at  Bradford  (Fritzing  Hall) : 
14.53  million  gallons  (55,000  cbm.)  of  sewage,  half  of  which  comes 
from  wool  washing  establishments,  are  treated  there  daily  by  the 
addition  of  sulphuric  acid.  The  sludge,  with  80  per  cent,  mois- 
ture, is  heated  to  212°  F.  (100°  C.)  by  steam  and  then  led  into  hot 
filter  presses,  where  the  grease,  which  has  been  separated  by  the 
sulphuric  acid,  is  pressed  out  with  the  water.  The  grease  sepa- 
rates from  the  water  and  is  refined.  In  1904  it  brought  a  revenue 
of  $29,300  (123,000  M.),  but  at  a  cost  of  $59,500  (250,000  M.), 
for  sulphuric  acid. 

The  pressed  cakes  contain,  in  addition  to  from  30  to  40  per  cent, 
of  water,  from  15  to  25  per  cent,  of  grease,  and  are  used  for  fuel; 
for,  as  has  been  stated,  they  are  not  suitable  for  use  as  a  fertilizer 
on  account  of  the  grease.  In  an  experimental  plant  one  part  of 
it  is  worked  over  for  grease  by  heating  the  sludge  cakes  in  retorts 
to  600°  F.  (315°  C.)  in  order  to  distill  the  grease.  This  is  then 
drawn  up  by  suction  and  condensed  when  it  is  almost  as  valuable 
as  when  first  obtained.  The  gas  water  obtained  is  treated  for 
ammonia  at  the  gas  plant  while  the  pulverized  residue,  containing 
1.5  per  cent,  nitrogen,  serves  as  a  fertilizer. 

If  the  liquid  wastes  from  wool  scouring  and  cloth  finishing 
plants  are  treated  separately,  the  grease  recovered  sometimes 
pays  for  the  entire  cost  of  purification.  It  is  of  advantage  to 
treat  the  sludge  from  several  factories  in  one  plant. 

The  great  disadvantage  in  treating  large  volumes  of  watery 
sludge,  as  at  Cassel,  especially  the  heating,  may  be  lessened  if 
the  grease  is  extracted  by  itself  from  the  main  body  of  the  sludge, 
as  is  done  with  the  floating  layer  in  the  Kremer  apparatus.  It  is 
impossible  to  prevent  a  part  of  the  grease  in  the  bottom  layer 
from  being  lost,  but  the  value  and  amount  of  grease  in  city  sew- 
age is  not  so  great  as  to  make  one  lay  great  stress  on  this  fact. 

It  is  much  more  important  to  keep  the  rest  of  the  sludge  as  free 
from  grease  as  possible,  as  it  is  then  more  suitable  as  a  fertilizer 
and  for  fuel  and  can  more  readily  be  rendered  inoffensive  by 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  109 

decomposition.  The  removal  of  grease  is  also  of  advantage  in 
connection  with  irrigation  and  final  biological  purification. 

The  layer  of  grease  in  the  Kremer  apparatus,  containing  an 
average  of  72  per  cent,  moisture  and  about  45  per  cent,  grease  in 
the  dried  material,  is  placed  in  a  perforated  vessel  for  further 
drying,  and  in  small  plants  can  be  given  directly  to  soap  manu- 
facturers, who  are  glad  to  get  it.  If  several  places  with  such 
arrangements  for  obtaining  the  grease  from  sludge  are  located 
near  each  other,  the  sludge  can  be  delivered  to  a  single  plant  for 
the  further  extraction  of  the  grease  withe  tetra-chloride  of  car- 
bon. With  cities  of  45,000  inhabitants  and  upward  an  independ- 
ent plant  for  working  over  the  sludge  is  warranted. 

A  thorough  extraction  of  the  grease  should  be  aimed  at  for 
economical  reasons;  for,  assuming  that  16.1  Ibs.  (7.3  kg)  per 
capita  reaches  the  sewage  each  year,  and  that  about  15,000,000 
people  in  Germany  live  in  cities  having  sewers,  about  121,000 
tons  (110,000,000  kg.)  of  grease  are  lost  annually. 

e.  VARIOUS  OTHER  METHODS  OF  DISPOSAL 

Aside  from  mixing  sludge  precipitated  by  lime  with  clay  to 
make  cement,  or  settled  sludge  with  loam  to  bake  into  bricks 
(these  methods  having  little  economic  value  on  account. of  the 
cheap  price  of  good  cement  and  brick,  but  being,  nevertheless, 
employed  to  a  considerable  extent)  we  will  here  mention  briefly 
methods  for  disposing  of  it  without  utilization. 

Sludge  is  frequently  used,  after  it  has  been  dried,  for  filling  in 
land,  sometimes  with  the  addition  of  slag  or  sand.  The  residue 
from  grit  chambers  is  particularly  adapted  to  this  purpose,  as  it 
contains  but  little  organic  matter;  and,  in  particular,  septic 
sludge,  as  it  has  but  little  heating  or  fertilizing  value,  and  is  not 
subject  to  offensive  decomposition.  Very  greasy  sludge  can 
often  be  used  in  this  way  only,  as  it  is  very  difficult  to  dry. 

Depressions  in  the  ground,  abandoned  sand  or  gravel  pits, 
the  dry  beds  of  streams  or  shores  of  rivers  (Elberfeld)  are  the 
best  dumping  grounds. 

This  method  of  disposal  is  much  used  at  Leipzig,  where  about 
163,000  cu.  yds  (125,000  cbm.)  of  wet  sludge  are  annually  dis- 
posed of  in  sludge  beds  of  about  21  acres  (8.5  hec.).  By  using 
an  old  stream  bed  lagoons  are  formed  by  earth  embankments, 
into  which  the  sludge  is  pumped  to  a  depth  of  15  ft.  (4.5  m.). 


110  SEWAGE  SLUDGE 

A  scum  forms  on  top  which  at  last  breaks,  sinks,  and  the  liquid 
rises  to  the  surface.  This  gradually  evaporates,  the  sludge  be- 
comes firmer  and  after  lying  for  some  years  is  in  part  carted  off. 

In  the  case  of  cities  on  the  coast  the  sludge  can  be  carried  to 
sea,  as  is  done  in  many  English  cities:  London,  Manchester, 
Southampton,  Dublin  and  Glasgow.  London,  at  Barking,  has 
the  best  arrangement  for  this.  The  sludge  is  there  pumped  into 
large  sludge  tanks,  as  its  removal  is  sometimes  interrupted  by 
fog,  and  the  water  rising  to  the  top  is  drained  off.  Six  steam- 
ships having  a  capacity  of  1300  cu.  yds.  (1000  cbm.)  each  are 
used  to  carry  the  daily  accumulation  of  about  7800  cu.  yds. 
(6000  cbm.)  of  sludge  to  about  50  miles  (80  km.)  beyond  the 
mouth  of  the  Thames,  where,  in  order  that  it  may  be  washed 
further  out,  it  is  dumped  shortly  after  low  tide. 

In  other  cities  vessels  with  a  capacity  of  from  780  to  1300 
cu.  yds.  (600  to  1000  cbm.)  are  used,  but  they  do  not  have  to  go 
so  far  out.  At  Dublin  the  sludge  is  dumped  but  2.2  miles 
(3.5  km.)  from  shore. 

Considered  from  a  sanitary  standpoint  this  method  is  free 
from  objection  as  the  sludge  is  at  once  rendered  harmless,  but 
its  valuable  constituents  are  lost  and  the  cost  of  disposal  is  quite 
large.  Including  loading  the  vessels,  interest  and  sinking  fund 
charges,  and  harbor  taxes,  this  amounts  to: 

7.2  cts.  per  ton  (.34  M.  per  long  ton),  at  Glasgow. 
8.9  cts.  per  ton  (.42  M.  per  long  ton),  at  Dublin. 
10.0  cts.  per  ton  (.47  M.  per  long  ton),  at  London  (Barking). 
12.3  cts.  per  ton  (.58  M.  per  long  ton),  at  Manchester. 
29.7  cts.  per  ton  (1.40  M.  per  long  ton),  at  Southampton,  where 
the  removal  is  done  by  contract. 

Septic  sludge  is  disposed  of  in  a  peculiar  way  at  Columbus. 
During  high  water  it  is  discharged  into  the  Scioto  River  which 
passes  the  purification  plant.  The  dilution  is  about  1 :  800. 
As  it  can  only  be  discharged  at  high  stages  of  the  river  the  sludge 
remains  in  the  tank  through  the  summer  (about  8  months). 
As  septic  sludge  is  not  offensive  no  objection  can  be  made  to 
this  method  so  long  as  no  deposits  accumulate  in  shallow  places. 
It  may  be  adopted  near  bodies  of  water  in  which  there  is  no  tide. 


CHAPTER  VI 

CONSIDERATIONS  REGARDING  THE  TREATMENT  AND  UTILIZATION 
OF  SLUDGE  IN  THE  CHOICE  OF  A  METHOD  OF  CLARIFICATION 

The  requirements  of  the  different  methods  for  removing  sludge 
from  tanks  and  drying  and  utilizing  it  have  already  been  con- 
sidered in  the  separate  foregoing  chapters,  as  well  as  the  advan- 
tages and  disadvantages  of  these  methods. 

In  order  to  utilize  the  product  profitably  it  is  necessary  in 
planning  the  work  to  give  due  consideration  to  the  method  of 
treatment  and  its  utilization. 

If  it  is  apparent  from  what  has  been  said  heretofore  that  the 
processes  for  the  treatment  and  utilization  of  sludge  have  been 
developed  to  such  an  .extent  that  there  is  no  necessity  to  reduce 
the  degree  of  purification  through  apprehension  of  a  troublesome 
sludge  burden;  yet  the  mistakes  that  have  been  made  due  to  an 
insufficient  study  of  the  sludge  question  and  an  underestimate  of 
its  importance  can  generally  be  rectified  only  by  a  large  outlay 
for  improving  the  mode  of  operation. 

The  composition  of  sludge  has,  up  to  this  point,  been  accepted 
as  something  definite.  If  it  is  not  possible  for  an  engineer  design- 
ing a  clarification  plant  to  radically  alter  the  character  of  the 
sludge,  he  can  at  least  modify  it  to  a  certain  extent  in  his  choice 
of  a  method  and  in  perfecting  its  details,  especially  those  features 
which  are  of  consequence  in  and  materially  facilitate  subsequent 
treatment  and  utilization. 

The  necessary  degree  of  clarification  is,  of  course,  the  funda- 
mental consideration  to  which  the  treatment  of  the  sludge  must 
defer.  But  by  measurement  of  the  sludge  the  solution  of  the 
problem  can  be  made  easier,  if  a  removal  of  the  foul  matter  from 
the  sewage  be  required  up  to  the  absolutely  necessary  degree, 
depending  upon  the  composition  of  the  water  into  which  it  is 
discharged  and  special  local  conditions;  for  with  the  reduction  of 
the  quantity  of  sludge  the  difficulties  of  disposal  will  diminish  as 
well  as  the  expense,  which  is  always  a  hinderance  to  a  hygienically 
desirable  and  thorough  purification.  The  more  complete  the 
purification  the  greater  the  amount  of  sludge  deposited. 

Ill 


112  SEWAGE  SLUDGE 

After  the  experience  of  the  past  in  the  utilization  of  sludge, 
(and  there  will  be  but  few  important  developments  in  the  future) 
no  more  foul  matter  will  be  separated  from  the  sewage  than  is 
demanded  for  sanitary  reasons,  without  attempting  to  further 
increase  the  volume  of  utilizable  sludge;  for  it  is  certain  that  with 
city  sewage  no  profit  can  be  secured  sufficient  to  cover  the  cost  of 
the  plant  and  furnish  additional  revenue. 

The  amount  of  sludge  to  be  looked  for  should  not  be  under- 
estimated, and  it  is  therefore  advisable  to  assume  the  maximum 
values  given. 

The  effort  of  the  designer  should  be  to  so  arrange  the  methods 
and  apparatus  for  removing  sludge  from  tanks,  for  reducing  the 
water  content  and  for  utilization  and  so  to  select  the  method, 
that  the  sludge  may  be  rendered  harmless  according  to  the 
demands  of  sanitation  and  at  the  least  cost.  He  should  examine 
carefully  into  methods  to  see  if  something  simpler  will  not  lead  to 
the  same  result,  or  whether  a  slight  increase  in  cost  may  not  be 
more  than  offset  by  the  avoidance  of  some  complicated  process. 

It  is  not  possible  without  much  repetition  to  mention  here  and 
describe  further  all  the  combinations  which  follow  from  con- 
siderations of  the  efficiency  of  clarification,  of  the  composition  of 
sewage  and  of  special  local  conditions,  such  as  available  room, 
relation  to  populous  districts  of  the  city,  etc.  Only  a  few  im- 
portant points  will  therefore  be  emphasized. 

The  principal  aim  should  be  to  obtain  little  sludge;  or,  what  is 
the  same  thing,  as  the  amount  of  dried  material  to  be  separated 
corresponds  to  the  clarification  effected,  sludge  with  as  little 
moisture  as  possible.  The  reason  for  this  is  stated  in  several 
places. 

If  the  degree  of  purity  required  is  high,  this  should  be  obtained, 
not  by  reducing  the  velocity  in  the  tanks,  but  by  passing  through 
contact  beds  or  over  irrigation  fields. 

The  construction  of  the  tanks  and  the  mechanical  appliances 
for  removing  sludge  during  operation  should  be  examined  pri- 
marily with  reference  to  the  amount  of  water  contained  in  the 
resulting  sludge,  and  especially  where  little  room  is  available  for 
drying  beds  and  where  the  general  conditions,  as  well  as  the  daily 
volume  of  sludge,  render  de-watering  by  centrifugal  machines 
impracticable,  the  disadvantages  of  removing  sludge  during 
interruption  of  operation  should,  in  some  cases,  be  accepted. 
Digestion  of  sludge  in  Emscher  tanks  may  at  times  be  an  advan- 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  113 

tage,  especially  in  small  installations,  where  there  is  a  great  saving 
in  labor,  as  the  sludge  need  not  be  removed  daily  and  this  is 
easily  done.  Still  simpler  is  the  treatment  of  sludge  in  septic 
tanks  of  ordinary  construction,  but  other  disadvantages,  such  as 
the  necessity  of  subsequent  cleaning,  as  well  as  the  storing  up  of 
great  masses  of  decomposing  matter  in  the  neighborhood  of 
inhabited  dwellings,  etc.,  often  stand  in  the  way  of  their  use. 

Sludge  should  be  obtained  in  such  a  condition  that  it  is  suitable 
for  drying  and  subsequent  utilization,  as  well  as  for  removal 
from  the  tanks.  For  this  reason  it  is  Necessary  to  free  it  from 
grease  and  cellulose.  This  is  best  accomplished,  especially  with 
greasy  sewage,  by  the  separation  of  these  substances  from  the 
sewage.  There  is  economy  in  recovering  the  grease  from  settled 
greasy  sludge.  The  preliminary  separation  of  the  grease  is 
especially  desirable  in  the  case  of  contact  beds,  sand  niters  and 
irrigation  fields. 

As  to  drying,  which  proceeds  more  rapidly  after  decomposition 
and  the  extraction  of  grease  in  the  open,  the  use  of  remote  sludge- 
drying  beds  is  often  to  be  recommended. 

With  artificial  methods  of  drying,  centrifugal  machines  seem 
open  to  much  improvement.  Here  it  is  a  question  of  reducing 
the  cost  of  the  apparatus  for  drying  by  simplifying  the  centri- 
fugal machine,  of  which  perhaps  the  one  constructed  on  the  prin- 
ciple of  a  cream  separator  might  be  improved. 

De-watering  by  filter-presses  need  be  considered  only  in 
special  cases,  as,  on  the  one  hand,  sludge  without  the  admixture 
of  other  material  is  not  adapted  to  this  treatment,  and  on  the 
other,  it  is  not  economical  to  add  this  merely  to  secure  a  sludge 
capable  of  pressing,  especially  as  the  volume  of  the  sludge  is 
thereby  increased. 

Many  experiments  have  been  made  in  the  utilization  of  sludge 
and  many  processes  have  never  passed  beyond  the  experimental 
stage,  so  that  it  is  as  yet  impossible  to  judge  of  them.  The 
author  has  gone  into  these  experiments  with  care,  as  it  is  only 
from  them  that  opinions  can  be  drawn  as  to  any  possible  develop- 
ments that  may  reasonably  be  looked  for  from  the  active  interest 
taken  in  the  matter. 

From  a  sanitary  standpoint  the  goal  to  be  aimed  at  is  the 
quickest  possible  removal  of  the  sludge  and  rendering  it  harmless 
and  this  is  likewise  true  regarding  its  disposal  and  drying.  As 
this  is  not  possible  without  expense,  economy  demands  the 

8 


114  SEWAGE  SLUDGE 

complete  utilization  of  the  valuable  materials  in  the  sludge  to 
reduce  the  cost  of  treatment,  or  at  least  the  expense  of  rendering 
it  harmless. 

If  the  first  case  seldom  occurs,  as  shown  in  Chapter  V,  there 
is  a  saving  in  giving  the  sludge  away  to  farmers  who  are  willing 
to  take  it.  Using  sludge  as  a  fertilizer  is  the  best  way  to  utilize 
the  valuable  ingredients,  which  are  constantly  increased  in 
amount  by  the  general  extension  of  sewerage.  The  nitrogen  in 
particular  is  conserved,  for  the  production  of  which  new  sources 
and  processes  are  being  constantly  sought,  while  by  burning 
this  is  lost.  The  extraction  of  grease  is  always  of  advantage 
when  used  for  agricultural  purposes. 

Irrigation  simplifies  the  treatment  of  sludge.  It  should,  there- 
fore, always  be  employed  where  the  conditions  render  it  possible. 
It  answers  the  same  purpose  with  respect  to  the  utilization  and 
rendering  inoffensive  of  the  impure  material  as  does  irrigation 
with  sewage,  which  is  the  best  method  as  to  its  removal  and 
utilization. 

Incineration  as  well  as  the  production  of  gas — which  latter 
process  can  and  will  be  improved — are  the  methods  of  utilization' 
indicated  where  clarification  is  brought  about  by  the  addition 
of  coal  or  peat. 

The  addition  of  coal  to  ordinary  sludge  to  facilitate  com- 
bustion seems  advisable  in  a  few  cases  only,  but  mixing  with 
house  sweepings  for  the  same  purpose  is  to  be  commended; 
only,  however,  in  case  the  sludge  cannot  be  disposed  of  in  some 
cheaper  and  simpler  way. 

Particular  emphasis  should  be  laid  on  the  greatest  simplicity  in 
the  method  of  utilizing  sludge. 

Extracting  grease,  according  to  the  present  status  of  methods 
employed,  can  only  be  done  with  economy  where  this  is  done 
by  itself  in  connection  with  particularly  greasy  sludge. 

A  brief  summary  of  the  principal  processes  in  their  relation 
to  the  treatment  and  utilization  of  sludge  is  given  here.  No 
consideration  is  given  to  their  various  advantages  or  disadvan- 
tages, as  has  been  done  generally  in  this  treatise. 

The  points  considered  are: 

a.  Composition  and  amount  of  sludge. 

b.  Methods  of  removing  sludge. 

c.  Adaptability  of  sludge  to  drying. 

d.  Possibility  of  the  utilization  of  sludge. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  115 

In  this  brief  space  the  various  modifications  of  method,  which 
are  very  great  in  sedimentation  processes,  cannot  be  treated. 
The  reader  is  referred  to  Chapters  III  and  IV. 

1.  Grit  Chambers. 

a.  Small  amount;  composition  and  moisture  contained,  de- 
pendent on  design  and  operation,  in  general  favorable. 

b.  Removal  of  sludge,  with  and  without  interruption,  without 
foul  odors. 

c.  Special  arrangements  for  de-watering  unnecessary. 

d.  Very  little  value,  chiefly  for  filling  land. 

2.  Mesh  and  bar  screens. 

a.  Small  amount  and  little  moisture.     Putrescible. 

b.  Simple  separation  in  fresh  condition. 

c.  Special  arrangements  for  de-watering  unnecessary. 

d.  Good  sale  as  a  comparatively  valuable  fertilizer. 

3.  Sedimentation  in  Tanks  and  Wells. 

a.  Large  volume  and  much  moisture,  especially  with  wells  and 
tanks  where  removal  occurs  during  operation.     Very  putrescible. 

b.  Removal  of  sludge  favorable  or  unfavorable,  according  to 
the  special  construction.     In  tanks,  usually  with  interruption  of 
operation  and  foul  odors;  in  wells  more  favorable  but  with  larger 
volumes. 

c.  Large  plant  for  drying  necessary,  as  sludge  is  not  always  of 
favorable  character.     Foul  odors  only  avoided  by  mechanical 
apparatus. 

d.  Useful  as  a  fertilizer,  but  less  so  than  from  screens  and  less 
readily  disposed  of  on  account  of  the  large  volume. 

4.  Chemical  Precipitation. 

a.  Very  large  volume.     Not  very  putrescible. 

b.  As  in  the  case  of  3b,  but  with  less  odor. 

c.  De-watering  practicable,  also  in  filter-presses;  little  nuisance 
from"  odors. 

d.  Of  very  little  use. 

5.  Lignite  process. 

a.  Very  large  volume  with  much  moisture.     Not  putrescible. 

b.  Removal  during  operation  without  foul  odors. 

c.  De-watering  in  filter-presses  or  in  the  air  quickly  and  with- 
out odor. 

d.  May  be  utilized  by  burning  or  conversion  into  gas. 

6.  Septic  tank  process. 

a.  Small  volume.     Little  moisture.     Non-putrescible. 


116  SEWAGE  SLUDGE 

b.  Little  nuisance  from  odor,  depending  upon  the  method  of 
removal.     Interruption  of  operation. 

c.  Favorable  consistency  for  drying. 

d.  Slight  utility,  as  calorific  and  fertilizing  values  are  reduced. 

7.  Emscher  Tanks. 

a.  As  with  6a. 

b.  Removal  during  operation  without  foul  odors. 

c.  As  with  6c. 

d.  As  with  6d. 

8.  Kremer  Apparatus. 

a.  As  with  3a,  but  more  favorable  composition  due  to  the 
extraction  of  grease. 

b.  Removal  during  operation,  when  sludge  with  less  moisture 
than  with  3b  is  obtained,  on  account  of  the  absence  of  grease. 

c.  For  the  same  reason,  favorable  consistency  for  de-watering. 

d.  Greasy    sludge    favorable    for    the    extraction  of   grease; 
sludge  at  the  bottom,  on  account  of  little  grease,  more  favorable 
for  fertilizer  than  ordinary  settled  sludge. 

Biological    treatment,  intermittent   filtration    and   irrigation 
are  omitted,  as  they  have  been  in  the  entire  treatise,  because 
sludge  here  plays  an  unimportant  role  and  its  treatment  depends 
entirely  upon  local  conditions  and  the  method  of  operation. 
•* 

CONCLUDING  REMARKS 

There  are  many  methods  for  the  treatment  and  utilization  of 
sludge  and  it  would  be  a  mistake  to  set  up  any  one  method  as 
best,  for  each  has  its  advantages  and  each  its  faults,  and  these 
have  weights  varying  with  the  local  conditions.  Certain  charac- 
teristics and  arrangements  may  prove  of  great  disadvantage  in  a 
large  city  while  well  adapted  to  a  small  town,  and  vice  versa. 

The  choice  must  be  made  to  suit  the  local  conditions  and  in  any 
given  case  these  must  make  necessary  corresponding  modifica- 
tions of  plan. 

If  this  is  not  observed  we  have  the  case  so  often  met  with  of  a 
method  adapted  to  one  place  and  working  admirably,  failing 
entirely  at  another.  The  blame  is  then  usually  laid — and  partly 
with  reason — to  an  overestimation  of  the  special  invention;  for  it 
is  frequently  observed  that  inventions  otherwise  useful  and  the 
results  of  experiment  lose  their  value  by  being  generalized,  and 
naturally  are  not  carried  out  under  the  same  conditions. 


TREATMENT  AND  UTILIZATION  OF  SLUDGE  117 

The  best  solution  of  the  sludge  question  can  only  be  found  by 
study  of  the  individual  case,  and  firmer  foundations  laid  by 
further  experiment  and  the  practical  application  of  the  various 
methods. 

A  sanitary  and  economical  treatment  and  utilization  of  sludge 
is  of  importance  for  each  plant,  and  one  can  but  agree  with 
Metzger  when  he  concludes  his  report  on  this  question  at  the 
International  Congress  for  Hygiene  and  Demography  at  Berlin 
in  1907  with  the  sentence:  "The  treatment  and  utilization  of 
sludge  are  of  such  great  importance  that  no  plant  should  be 
completed  until  all  the  questions  of  subsequent  treatment  have 
been  finally  answered  and  settled  byf  avoiding  the  conditions 
known  in  principle  to  be  evil." 


THE  DRYING  OF  SLUDGE 

A  Report  from  the  Sewerage  Division  of  the  Emscher  Association.      Kgl.    Baurat 
Middeldorf,  Chief  Engineer.      Dr.  Ing.  Imhoff ,  Division  Superintendent. 


BY 

DR.  ING.  FR.  SPILLNER 

ESSEN-RUHR 

TRANSLATED   BY 

KENNETH  AND  ROSE  S.  ALLEN 


INTRODUCTION 

The  Sludge  Question. — The  difficulties  encountered  to-day  in 
treating  sludge  lie  less  in  the  methods  employed  than  in  the 
characteristics  of  the  sludge.  The  liquid  masses  of  sludge 
obtained  from  sewage  can  neither  be  utilized  nor  left  at  the  plant, 
and,  as  a  rule,  cannot  be  removed  without  great  cost. 

In  England,  where  the  question  of  sewage  purification  was 
first  considered  on  a  large  scale,  on  account  of  the  small  volumes 
of  water  available  into  which  it  could  be  discharged,  and  where 
every  large  city  has  its  disposal  plant,  the  sludge  question  has 
become  most  urgent.  The  editor  of  The  Surveyor  has  recently 
said:1 

"The  solution  of  the  sludge  problem  is  the  most  pressing 
question  of  the  day  and  a  little  practical  assistance  in  this  direc- 
tion from  our  scientists  would  be  of  much  greater  value  than 
all  the  learned  dissertations  on  theories  and  doctrines  with  which 
we  have  been  favored  in  recent  years. " 

The  well-known  authority,  Barwise,  has  expressed  a  similar 
opinion.2 

That  the  importance  of  these  questions  has  been  recognized 
in  Germany  for  some  time  was  shown  by  the  reports  presented 
at  the  Fourteenth  International  Congress  of  Hygiene  and  Demog- 
raphy, held  in  Berlin,  City  Engineer  Metzger  (Bromberg)  said 
there:3 

"The  many  attempts  to  purify  city  sewage  in  the  past,  with 
the  great  activity  that  has  been  shown  in  relation  thereto, 
would  have  led  to  better  results  if  the  resulting  sludge  were  not 
a  troublesome  accompaniment,  interfering  with  any  satisfactory 

solution In  most  towns the  removal  of  sludge  was 

a  bete  noir,  and  many  plants  would  have  produced  a  more  favor- 

1  The  Surveyor,  1909,  No.  886,  p.  27. 

2  Sydney  Barwise,  Med.  Officer  of  Health.      The  Sanitary  Record,  1909,  p.  122. 
3Ges.  Ing.,  1908,  No.  4,  p.  50-53. 

121 


122  SEWAGE  SLUDGE 

able  impression  if  it  had  not  been  for  the  mountains  of  detritus. 
It  was  realized  at  last  that  all  attempts  to  utilize  sludge  had 
led  to  unsatisfactory  results. 

"The  removal  and  utilization  of  sludge  is  of  such  importance 
that  no  plant  should  be  completed  until  all  questions  of  handling 
sludge  while  avoiding  the  known  nuisances  are  finally  decided/' 

The  principal  trouble  lies  in  the  large  amount  of  water,  which 
is  so  difficult  to  remove.  After  it  has  been  removed  from  the 
plant  the  sludge  contains  at  least  70  per  cent.,  usually  90  to  95 
per  cent.,  and  occasionally  99  per  cent,  of  moisture  and  is  a 
liquid  mass.  Every  manipulation  of  the  material,  whether 
with  the  view  of  utilization,  storage  or  removal,  is  rendered  more 
difficult  and  expensive  by  this  ballast  of  water. 


THE  DRYING  OF  SLUDGE 

CHAPTER  I 


NECESSITY  OF  DRYING 

Utilization. — The  value  of  sludge  lies  in  the  organic  material 
contained  in  it.  Heretofore  this  has  been  used  as  a  fertilizer,  by 
recovering  the  contained  grease  or  by  converting  its  calorific 
value  into  heat. 

Fertilizing  Value. — Its  employment  as  a  fertilizer  has  been 
most  widespread.  In  small  plants  in  an  agricultural  region  the 
sludge  can  be  disposed  of  and  it  is  often  possible  to  secure  a  small 
revenue  therefrom,  especially  where  the  soil  is  dry  and  sandy. 
In  large  cities  the  farmers  will  seldom  take  the  sludge  when  it  is 
wet.  They  usually  demand  a  spadable  product. 

The  following  example  shows  what  difficulties  were  encoun- 
tered in  disposing  of  wet  sludge  even  a  few  years  ago. 

The  City  of  Edinburgh,1  Scotland,  in  the  year  1892,  sent  1521 
circulars  to  farmers  in  the  neighborhood  asking  for  proposals 
to  take  58,100  tons  (51,900  long  tons) — i.e.,  the  accumulation 
of  a  half  year. 

Only  47  bids  were  received  and  all  with  the  condition  that  the 
city  pay  for  transportation,  some  even  demanding  a  bonus  for 
each  ton  removed. 

Other  large  cities  without  suitable  land  for  irrigation  fields 
have  had  a  similar  experience.  The  farmers  either  refused  to 
take  the  wet  sludge  or  demanded  compensation. 

Dried  sludge  is  more  suitable  for  use  as  a  fertilizer,  but  the  cost 
is  usually  greater  than  the  amount  received.  There  are,  to  be 
sure,  some  exceptions.  In  Kingston-on-the-Thames2  (near 
London) ,  sludge  obtained  by  the  A.  B.  C.  process  (alum,  blood, 
coal)  and  then  dried  by  heat,  was  sold  as  " Native  Guano"  for 
about  $15.10  per  ton  (70  M.  per  1000  kg.),  a  price  strangely  in 
contrast  to  all  other  results. 

1  Zentralblatt  der  Bauverwaltung,  1892,  No.  22,  p.  240. 

2  Douglas,  Sanitary  Record,  Vol.  XL,  No.  973,  p.  424. 

123 


124 


SEWAGE  SLUDGE 


Charlottenburg1,  where  an  annual  amount  of  about  23,636 
cu.  yds.  (18,070  cbm.)  of  spadable  sludge  is  obtained  at  the  irriga- 
tion fields  at  Gatow,  receives  but  5.3  cts.  per  cubic  yard  (30  pfg. 
per  cbm.).  More  than  9.1  cts.  per  cubic  yard  (50  pfg.  per  cbm.) 
is  seldom  paid  and  this,  usually,  only  during  the  first  years  of 
operation.  As  soon  as  the  farmers  realize  that  the  sludge  must 
be  disposed  of  they  will  offer  nothing;  and  sometimes  even 
demand  compensation. 

At  Frankfort-on-the-Main  the  greatest  difficulty  was  experi- 
enced merely  to  get  rid  of  the  spadable  sludge.  The  large  con- 
sumers demanded  pay  for  cartage  and  the  furnishing  of  labor  for 
loading  and  unloading. 

At  Cassel2  sludge  composted  with  sweepings  is  given 'away. 

At  Leipzig,3  also,  where  sludge  is  obtained  by  precipitation 
with  iron  salts  and  is  comparatively  cheap  to  dry,  and  where 
12  cts.  (50  pfg.)  per  load  is  received,  or,  from  the  lessees  of  the 
city,  6  cts.  (25  pfg.)  [where  they  do  their  own  loading,  6  cts. 
(25  pfg.)  less]  there  have  been  paid  for  the  cartage  of  dried  sludge 
from  its  places  of  deposit  and  received  therefor:4 


Paid  for  cartage 

Received  for  sludge  and 
screenings 

Voar 

0 

$ 

M. 

$ 

M. 

1905         .... 

5,509.51 

23,149  22 

26.70 

112.20 

1906  

7,373.75 

30,982.15 

76.87 

323.00 

1907  

7,308.64 

30,708.56 

81.40 

342.00 

As  the  expense  of  cartage  from  the  drying  beds  represents  only 
a  portion  of  the  costs  chargeable  to  sludge — there  being,  in 
addition,  the  establishment  and  maintenance  of  the  drying  beds, 
erecting  a  structure  for  loading,  which  appears  in  the  budget  for 
the  year  1905,  $2,483.68  (10,435.64  M.),  besides  interest  and 

1  Geh.  Med.-rat  Prof.  Dr.  Salomon,  Die  stadtische  Abwasserbeseitigung  in  Deutschland. 
Vol.  II,  p.  186. 

2  Stadtbaurat  Hopfner  and  Dr.  Paulmann,  Die  Schmutzwasserreiningungsanlange  der 
Stadt  Kassel.      Vierteljahrsschrift  fur  gerichtL     Medizin  und  dffentliches  Sanitdtswesen, 
1900. 

3  Inspected  Dec.  7,  1908. 

4  Official  report  of  the  city  of  Leipzig. 


DRYING  OF  SLUDGE  125 

sinking  fund  charges — this  example  shows  how  little  we  can  count 
on  a  profit  from  the  fertilizing  value  of  sludge. 

One  of  the  principal  difficulties  in  the  use  of  sludge  for  farming 
purposes  is  the  fact  that  fertilizers  are  usually  used  only  during 
the  winter  months.  As  soon  as  it  became  possible  to  produce 
from  the  sludge  a  firm  fertilizer  the  feasibility  of  its  transporta- 
tion was  increased.  The  city  of  Frankfort-on-the-Main  has 
made  experiments  in  this  direction,  and  manufactured  poudrette 
in  order  to  ascertain  the  cost  of  the  process.  Drying  was  neces- 
sary for  this  also.  It  was  found,  however,  that  even  when  it 
could  be  sold  at  market  prices  the  excess  in  cost  of  manufacture 
amounted  to  $71,400  (300,000  M.)  per  annum. 

The  farmers  object  to  using  fresh  sludge  on  their  fields  con- 
stantly because  of  the  nuisances  which  arise  therefrom.  Some- 
times it  causes  vermin  to  appear,  it  frequently  brings  the  seeds 
of  weeds  and  always  much  grease  and  cellulose.  The  latter 
makes  the  soil  slimy,  as  has  often  been  proven.1  Efforts  were 
then  made  to  separate  and  destroy  these  materials,  or  at  least  to 
utilize  them,  especially  the  grease. 

Utilization  of  Grease. — Sludge  contains  grease  in  varying 
amounts.  It  comes  in  sewage  from  wash  water  and  other 
domestic  refuse,  from  slaughter  houses  and  from  soap  suds,  and 
hence  is  found  in  the  sludge.  Based  upon  the  dried  material 
there  was  found  in  Liittich2 18  per  cent.,  in  Cassel3  15  per  cent.,  in 
Frankfort4  16.71  per  cent.  In  Harburg5  14.2  per  cent,  in  raw 
sludge  and  8.5  per  cent,  in  centrifuged  sludge.6 

No  use  has  as  yet  been  discovered  for  the  grease  obtained  from 
sludge.  The  large  amount  of  water  contained  renders  it  too 
costly.  The  experiment  at  Cassel  is  well  known.  At  a  cost  of 
$47,600  (200,000  M.)  a  reduction  plant  was  built  where  grease 
was  recovered  by  the  use  of  benzine  after  most  of  the  water  had 
been  removed  by  filter  presses,  and  the  residue  was  used  as  a 

1  Dr.    L.    Haack,    Berlin.     Verwertung   und    Beseitigung   des   Klarschlammes  aus   den 
Reinigungsanlagen  stadtischer  Abwasser.  Gesundheitsingenieur.      1908,  p.  53. 
.  2  Dr.  Lacomble,  Le  scat  des  matieres  grasses,  etc.    Revue  de  Phygiene  et  de  police  sani- 
taire.    .28  No.  10. 

3  Stadtbaurat  Hopfner  and  Dr.  Paulmann,  Mittellungea  aus  der  Kgl.  Prufungsanstadt  fur 
Wasservers  orgung  und  Abwasserbeseitigung  zu  Berlin.     Pub.  by  Aug.  Hirschwald.    Vol.  I. 

4  Dr.   Bechold  and  Dr.   Voss;  Zur  Fettgewinnung  aus  Abwassern.     Zeitschr.  f.  angew. 
Chemie,  1908,  p.  1318. 

5  Regierungsbaumeister  Reichle  and  Prof.  Dr.  Thiesing.      Versuche  mit  dem  Schlamm- 
schleuder    apparat    Schafer-ter    Meer.     Mitteilung    a.   d.  Kgl  Priif.  Anst.   f.  Wasservers, 
usw.     Vol.  X,  p.  190. 

0  Ditto,  p.  154. 


126  SEWAGE  SLUDGE 

fertilizer.  From  65  cu.  yds.  (50  cbm.)  of  wet  sludge  61/2  cu.  yds. 
(5  cbm.)  of  dry  sludge  was  expected,  from  which  1650  Ibs.  (750 
kg.)  of  crude  grease  and  10,750  Ibs.  (4885  kg.)  fertilizing  sludge 
was  looked  for.  The  latter  was  estimated  at  32  1/2  cts.  per 
100  Ibs.  (3  M.  per  100  kg.)  to  be  worth  $34.88  (146.55  M.).  From 
the  crude  grease  990  Ibs.  (450  kg.)  refined  grease  was  realized, 
which  was  estimated,  at  $4.87  per  100  Ibs.  (45  M.  per  100  kg.)  to 
be  worth  $48.20  (202.50  M.) ,  and  495  Ibs.  (225  kg.)  of  tarry  residue 
which  was  estimated  at  41.6  cts.  per  100  Ibs.  (2  M.  per  100  kg.). 

The  total  revenue  from  65  cu.  yds.  (50  cbm.)  wet  sludge  was 
therefore  estimated  at  $88.14  (353.55  M.).  In  spite  of  this 
the  expenses  were  greater  than  the  receipts,  and  the  plant  was 
abandoned  and  taken  down.  One  reason  for  the  failure,  aside 
from  the  high  cost  of  drying,  was  that  the  grease  obtained  was 
not  marketable,  on  account  of  its  disagreeable  odor. 

At  Frankfort  Bechold  and  Voss  have  made  a  very  thorough 
study  of  the  question  of  grease  recovery,  accompanied  by  many 
experiments.  They  avoid  expensive  drying  and  extract  the 
grease  from  the  wet  sludge  with  benzine  after  treating  with  acid 
and  heating  to  140°  to  158°  F.  (60°  or  70°  C.) .  Aside  from  the  ex- 
perimental plant,  which  is  said  to  have  given  good  results,  the 
method  has  not  as  yet  been  introduced. 

Experiments  are  also  now  being  made  by  Dr.  Grossmann  at 
Oldham,  near  Manchester,  to  demonstrate  the  practicability  of 
his  method  of  recovering  grease  from  sludge  by  distillation  with 
steam.  The  method  of  Dr.  Grosse-Bohle  (Cologne)1  by  which 
sludge  was  heated  to  122°  F.  (50°  C.)  to  obtain  the  grease  from 
the  resulting  scum  has  not  yet  been  successful  on  a  large  scale. 

If  a  substance  contains  15  per  cent,  of  grease,  as  is  the  case 
with  the  dried  matter  in  fresh  sludge,  as  shown  by  the  analyses 
already  given,  the  possibility  of  a  profit  is  assured.  As  sludge 
contains  from  90  to  95  per  cent,  of  water,  however,  these  figures, 
based  on  the  wet  material  as  delivered,  will  be  materially  de- 
creased. 15  per  cent,  of  grease  in  the  dried  material  is  only  1 . 5 
per  cent  in  the  sludge  90  per  cent,  moisture,  and  8.5  per  cent, 
(centrifuge  sludge  from  Harburg2)  is  only  2.34  per  cent,  in  sludge 
with  72.5  per  cent,  moisture.  The  water  is  here  again  the  great 
stumbling  block  to  utilization.  The  prospect  of  utilizing  the 

1  Hofrat  Dr.  Friedrich,  Kulurtechnischer  Wasserbau.     2nd  Ed.  Vol.  II,  p.  482. 

2  Inspected  May  9,   1908.     See  also  Dr.  Ing.  Bruno  Heine,  Das  Kanalsationswerk  der 
Stadt  Copenick.     Gesundheit,  1909,  No.  19,  23. 


DRYING  OF  SLUDGE  127 

grease  contained  would  be  greatly  increased  by  some  cheap 
method  of  drying. 

Utilizing  the  Calorific  Value  of  Sludge. — The  drying  of  sludge 
is  absolutely  necessary  to  obtain  its  calorific  value.  Two 
methods  of  doing  this  have  been  attempted:  by  direct  com- 
bustion and  by  conversion  into  gas.  Sludge  without  some 
addition  is  seldom  combustible,  even  when  thoroughly  dried. 
Experiments  made  and  the  plants  constructed  so  far  deal  ex- 
clusively with  mixtures  with  dust  or  else  sludge  precipitated 
with  combustible  material  (coal  or  peat).  At  Copenick1  and 
Potsdam2  lignite  sludge  obtained  by  Degener's  method  (ground 
lignite  and  sulphate  of  alumina  were  thoroughly  mixed  with  the 
sewage  to  be  clarified)  was  used.  In  Copenick  the  preliminary 
drying  is  accomplished  in  sludge  tanks,  the  next  step  takes  place 
under  a  large  shed  after  removal  and  it  is  finally  dried  out  at  the 
feed  openings  of  incinerators.  The  heat  generated  is  transformed 
into  electricity  by  steam  power.  In  Potsdam  the  sewage  is 
clarified  in  towers  (Rothe-Degener  process)  and  the  precipitated 
sludge  is  dried  in  filter-presses,  made  into  briquettes  and  in  part 
sold  as  fuel  and  in  part  used  in  the  generation  of  electricity  as  at 
Copenick.  In  both  cases  coal  is  added  to  the  sludge. 

Efforts  to  obtain  the  calorific  value  from  such  lignite  sludge 
have  frequently  been  made,  e.g.,  by  Heine3  in  the  central  office 
of  the  General  Electric  Co.,  at  Berlin,  and  by  Schury  and  Bujard4 
(peat  clarified  sludge)  which  have  furnished  good  results. 

Gohring,5  as  well  as  Reichle  and  Dost,6  have  published  ex- 
periments on  the  conversion  of  sludge  into  gas.  Other  experi- 
ments of  the  kind  have  been  made  at  Manchester.7  No  attempts 
to  introduce  this  process  on  a  large  scale  have  been  made.8 

1  Idem. 

2  Inspected  May  8,  1908,  and  Nov.  28,  1908. 

3  Dipl.-Ing.    Bruno  Heine,   Neber  die  Erzeugung  electrischer  Energie  mit  Hilfe   von 
Kanalisationsklarschlamm.     Dissertation.     Tech.  Hochschule,  Berlin. 

4  Regierungsbaumeister  Schury  und   Dr.    Bujard,    Torfbreiklarversuch  der  Stadt  Stutt- 
gart in  der  Kohlebrei  klaranlage  zu  Tegel.     Mitteilungen  a.  d.  Kgl.  Priif.-Aust.  f.  Was- 
servers.  usw.  Vol.  VIII,  p.  143. 

5  C.  F.  Gohring.     Beitrage  zur  Reinigung  von  stadtischem  und  Fabrikabwasser.    Leipzig, 
1904. 

6  Regierungsbaumeister    Reichle    und    Dr.    Dost.     Ueber    Schlammverwertung    durch 
Vergasung,  insbesondere  beim  Rothe-Degenerschen  Kohlbreiverfahren.     Mitteilung  a.  d. 
Kgl.  Priif.  f.  Wasservers.  u.  Abwasserbes,  Vol.  VIII,  p.  146. 

7  Baurat  Bredtschneider  und  Dr.  Thumm.     Die  Abwasserreinigung  in  England.     Mit- 
teilung a.  d.  Kgl.  Pruf.-Anst.  f.  Wasserv.  usw.      Vol.  Ill,  p.  93. 

8  Aufsatze  tiber  Schlammverbrennung  und  Vergasung  siehe  R.  Frank,   Vergasung  von 
Abwasserklarschlamm.    Ges.  Ing.  1907,  p.  465,  and  Koschmieder,  Tech.  Gem.  1905,  No 
19. 


128  SEWAGE  SLUDGE 

Sludge  is  mixed  with  sweepings  and  burned  in  several  places, 
as  at  Bury/  where  the  sludge  dried  by  filter  presses  is  mixed 
with  street  sweepings  in  the  proportion  of  1  :2  and  burned  in 
Horsfall  furnaces.  This  is  also  done  at  Hyde.2  A  large  plant 
of  this  kind  has  recently  b^en  put  up  at  Frankfort,  but  the  proc- 
ess does  not  appear  to  be  economical.  An  annual  deficit  in 
operating  expenses  of  $17,612  (74,000  M.)  was  expected  at  the 
outset3  as  compared  with  the  former  minimum  outlay  for  sludge 
disposal,  $6545  (27,500  M.).  The  burning  of  sludge  has  also 
been  planned  at  Pforzheim.4 

The  advantages  of  this  method  are  undeniable.  One  is  no 
more  dependent  on  the  good  will  of  the  former.  The  disposal 
of  the  sludge  is  assured  and  the  nuisance  overcome.  No  other 
method  is  so  sanitary.  Its  general  use  is,  however,  prevented 
by  the  cost  of  drying.  Here,  more  than  with  other  methods, 
it  is  clear  that  the  sludge  question  is  essentially  a  question  of 
drying. 

Removal.  Dumping  at  Sea. — As  long  as  drying  is  so  expensive, 
the  burning  of  sludge  may  be  classed  with  those  methods  of 
disposal  in  which  no  profit  can  be  expected,  and  of  these,  it  is 
the  most  costly.  According  to  the  Royal  Commission  on 
Sewage  Disposal5  the  cost  of  drying  and  burning  is  estimated 
at  35.7  cts.  (1.50  M.)  for  one  long  ton  (1015  kg.)  of  wet  sludge, 
while  carrying  it  to  sea  costs  but  about  3.6  cts.  (0.15  M.)  when 
the  distance  is  not  too  great.  These  methods  are  in  use  in 
London  (Barking  and  Crossness6),  at  Manchester7  and  at  Salford,8 
and  are  proposed  for  several  other  places,  e.g.,  Belfast.  Cities 
not  situated  directly  on  the  sea  can  use  this  method  (those 
directly  on  the  shore  discharge  their  sewage  untreated  into  the 
water,  as  at  Copenhagen)  provided  there  is  some  direct  waterway 
to  the  sea  and  that  the  distance  is  not  too  great;  otherwise  the 

1  Dr.    Ing.    Schiele.     Abwasserbeseitigung    von    Gewerben,    etc.     Mitteilungen.    a.    d. 
Kgl.  Priif.-Anst.  f.  Wasserv.  usw.      Vol.  XI,  p.  172. 

2  Idem,     Vol  XI,  p.  781. 

3  Stadtrat  Kolle  und   Stadtbauinspector  Uhlfelder.      Denkschrift  iiber  den   Bau  einer 
Mullverbrennungsanstalt  zur  Unschadlichmachung  der  Hausabfalle  und  des  Klarbecken- 
schlammes  in  Frankfort  a.  M.,  p.  25. 

4  Stadtbaumeister  Herzberger  und  Dipl.-Ing.   Morave.  Projekt  einer  Mullverbrennung- 
sanstalt mit  Klarschlammtrocknung  fur  die  Stadt  Pforzheim.     Ges.-Ing,  1907,  No.  40,  p. 
649. 

5  Royal  Commission  on  Sewage  Disposal.     5th  Rep.  London,  1908.     Wyman  and  Sons, 
Ltd. 

6  Inspected  May,  1907. 

7  Inspected  Sept.  14,  1909. 

8  Inspected  Sept.  14,  1909. 


DRYING  OF  SLUDGE  129 

charges  for  transportation  are  excessive.  At  London  the  dis- 
tance is  about  62  miles  (100  km.),  at  Manchester  and  Salford 
50  miles  (80  km.).  London  has  a  whole  fleet  of  sludge  steamers, 
each  of  which  holds  1120  tons  (1000  long  tons)  and  costs  $142,800 
(000, 000  M.),  besides  large  iron  tanks  where  the  sludge  is  stored 
until  the  arrival  of  the  steamer.  Manchester  and  Salford  have 
each  one  steamer.  Three  trips  a  week  are  made  on  an  average. 
A  670  ton  (600  long  ton)  steamer  cost  $57,120  (240,000  M.)  in 
1895  for  Salford.  Removal  there  costs  about  17  cts.  per  ton 
[80  pfg.  per  long  ton  (1015  kg.)]  of  wet  sludge;  in  Manchester, 
in  1902-3,  16.6  cts.  per  ton  (78  pfg.  per  long  ton).1 

Influence  of  Drying  on  the  Cost  of  Removal. — The  following 
considerations  show  how  the  removal  of  water  reduces  the 
cost  of  every  method  of  disposal  where  there  is  a  question  of 
transportation:  if  95  per  cent,  is  reduced  to  90  per  cent,  there 
will  be  9  parts  of  water  to  1  part  of  dried  matter,  where  before 
there  were  19.  There  is,  then,  only  1/2  the  original  quantity 
to  dispose  of;  with  80  per  cent.,  1/4;  with  70  per  cent.,  1/6; 
with  60  per  cent.,  1/8;  and  with  50  per  cent.,  1/10.  As  fresh 
sludge  with '70  per  cent.,  and  septic  sludge  with  60  per  cent, 
moisture,  has  attained  a  consistency  where  it  can  be  handled, 
like  damp  earth,  and  can  be  carried  in  any  kind  of  wagon,  drying 
to  this  point  has  a  great  advantage. 

Placing  on  Land. — Drying  is  usually  necessary  when  land 
instead  of  the  sea  is  used  as  a  dumping  ground,  partly  on  account 
of  the  smaller  cost  for  transportation,  which  is  much  greater  by 
land  than  by  sea,  partly  to  avoid  the  foul  odors  from  the  piled 
up  sludge.  Wet  sludge,  especially  from  sedimentation  tanks, 
emits  a  very  foul  smell  after  a  short  time,  which  can  be  recog- 
nized for  a  long  distance.  By  drying  until  it  can  be  spaded 
this  difficulty  is  overcome. 

This  is  done  at  Hanover2  after  de-watering  by  a  centrifugal 
machine. 

The  drying  of  sludge  js  necessary  to  reduce  the  cost  of  trans- 
portation and  to  obtain  its  calorific  value.  It  frequently  renders 
it  available  as  a  fertilizer.  All  nuisances  and  costs  occasioned 
by  sludge  would  be  reduced  to  a  minimum  if  a  dry  product 
could  be  obtained.  In  the  main,  therefore,  the  sludge  question 

1 G.  Ash  ton  (Manchester).  Disposal  and  Utilization  of  Sewage  Sludge.  Surveyor, 
1904,  p.  320. 

2  Inspected  Nov.  18,  1907,  May  6,  1908,  and  Nov.  21,  1908. 


130  SEWAGE  SLUDGE 

is  a  question  of  drying.     Sludge,  however,  has  a  great  aversion 
to  drying. 

METHODS  OF  DRYING   DIFFERENT  KINDS  OF  SLUDGE 

Classification  of  Methods. — Different  methods  of  clarification 
produce  different  kinds  of  sludge.  These  show  very  different 
characteristics,  especially  in  the  matter  of  drying. 

Mechanical  methods  of  clarifying  can  be  divided  into  "fresh 
methods"  and  "septic  methods,"  as  regards  the  final  condition 
of  water  and  sludge.  In  the  first,  putrefaction  is  prevented  in 
order  to  keep  the  sewage  fresh;  in  the  second  it  is  promoted  to 
digest  the  sludge.  Between  the  two  is  the  Emscher  method, 
or  similar  ones,  where  the  sewage  is  kept  fresh,  but  the  sludge 
is  subjected  to  thorough  decomposition. 


CHAPTER  II 
DRYING  OP  FRESH  SLUDGE 

Source. — Arrangements  used  in  the  fresh  methods  (sedimenta- 
tion and  precipitation)  are  in  general  a  widening  of  the  cross- 
section  of  the  channel,  by  which  the  current  is  retarded.  Mater- 
ial which  is  capable  of  settling  is  thus  given  the  opportunity. 
These  arrangements  are  in  the  form  of  tanks  (horizontal  move- 
ment of  the  water),  or  wells  or  towers  (vertical  movement). 
The  water,  as  a  rule,  takes  from  1  to  4  hours  to  pass  through. 
Sludge  which  settles  on  account  of  its  weight  (plain  sedimenta- 
tion) or  whose  separation  is  increased  or  facilitated  by  the  addi- 
tion of  chemicals  to  the  sewage,  causing  the  flacculent  matter 
to  settle  (precipitation  method)  is  removed  before  it  putrefies. 
Tanks  for  this  purpose  are  usually  entirely  emptied.  Wells 
and  towers  are  frequently  so  arranged  that  the  sludge  can  be 
removed  under  water  by  pumps  or  vacuum  receivers  without 
interruption  of  operation. 

Characteristics  of  Fresh  Sludge. — Fresh  sludge  is  marked  by 
the  great  amount  of  water  contained.  This  is  90  to  95  per  cent, 
on  an  average,  according  to  data  given  by  Bredtschneider  and 
Thumm,1  Imhoff,2  Dunbar,3  and  von  Schiele.4  Reports  on  the 
examination  of  sludge  from  different  plants  confirm  these  figures. 
Thus  in  Cologne  Grosse-Bohle5  found  between  91.34  per  cent, 
and  95.57  per  cent.,  and  Tillmans6  found  at  Frankfort-on-the- 
Main  an  average  of  91.07  per  cent.  I  have  never  found  less 
than  90  per  cent.,  but  often  more — up  to  96  per  cent.  Honig7 
found  in  Brimn  the  surprising  amount  of  99  per  cent. 

1  Die  Abwasserreinigung  in  England.     Mitteilungen  a.  d.  Kgl.  Priif.-Anst.  f.  Wasserv. 
usw,  Vol.  III. 

2  Regierungsbaumeister     Dr.     Ing.    Imhoff.     Die     biologische     Abwasserreinigung     in 
Deutschland.      Mitteil.  a.  d.  Priif.  Anst.  f.  Wasserv.  usw,  Vol.  VII. 

3  Leitfaden  fur  die  Abwasserreinigungsfrage. 

4  Abwasserbeseitigung  von  Gewerben,  etc.     Mitteilungen  a.  d.  Kgl.  Priif. -Anst.  f.  Was- 
serv. usw,  Vol.  XIII. 

5  Die  Probeklarunglage  zu  Koln-Niehl.     Mitteilungen  a.  d.  Kgl.     Pruf.-Anst.  f.  Wasserv. 
usw,  Vol.  IV. 

6  Die  Klaranlage  in  Frankfurt  a.  M.  Wasser  und  Abwasser,   Vol.  I,  p.  320. 
7Gewinnung  und  Verwertung  von  Stadtischem  Klarschlamm.     Ges.  Ing.,  1910.     Nos.  1 

and  2. 

131 


132  SEWAGE  SLUDGE 

On  account  of  this  large  amount  of  water,  fresh  sludge  from 
city  sewage  is  a  very  liquid  material.  It  can  be  concentrated 
after  standing  several  days,  when  roily  water  appears  at  the 
surface,  but  it  is  seldom  possible  to  reduce  the  moisture  much 
below  90  per  cent.  More  than  half  of  the  5  per  cent,  to  10  per 
cent,  dried  substance  consists  of  organic  matter.  This  decom- 
poses very  readily,  giving  a  characteristic  indefinable  pene- 
trating odor.  The  nuisances  arising  from  this  odor  prevent  the 
storing  and  further  treatment  at  the  plant,  when  the  neighbor- 
hood has  become  built  up.  Disposal  plants  have  therefore  been 
placed  at  a  great  distance  from  the  source  of  the  sewage,  and 
where  for  any  reason  this  has  not  been  done,  the  inhabitants 
have  raised  much  objection.  In  Braunschweig,1  e.g.,  the  opera- 
tion of  a  plant  where  fresh  sludge  was  produced,  and  where  it 
was  to  be  dried,  had  to  be  abandoned.  Since  then  the  sewage 
has  been  spread  on  irrigation  fields. 

At  Frankfort-on-the-Main2  the  odors  were  overcome  by 
spraying  with  tarry,  floating  oils  (facilol  and  Belloform)  and 
also  covering  the  surface  with  peat. 

At  other  places,  e.g.,  in  Remscheid,3  Langensalza  and  Aschers- 
leben4  (wells  of  the  Mairich  system)  attempts  have  been  made 
to  reduce  the  odor  by  placing  the  sludge  beds  on  hills  and  pump- 
ing the  sludge  up  at  a  great  cost.  The  odor  was  thus  dispelled 
by  the  wind. 

Quantities  of  Fresh  Sludge. — The  unpleasant  features  con- 
nected with  sludge  are  so  much  felt  because  of  the  large  quan- 
tities which  accumulate.  According  to  Imhoff,5  0.32  gallons 
(1.2  liters)  of  sludge  with  95  per  cent,  moisture  per  inhabitant 
per  day  accumulate  in  plants  of  large  cities  where  the  sedimen- 
tation process  is  used.  For  a  city  of  100,000  inhabitants  this 
gives  an  annual  amount  of  56,000  cu.  yds.  (43,000  cbm.)  of 
sludge  which  must  be  cared  for. 

Drying. — As  shown  above,  since  drying  reduces  the  volume, 
it  is  almost  always  a  necessary  preliminary  in  caring  for  sludge. 
Fresh  sludge  is  obstinately  opposed  to  drying.  The  main  diffi- 
culty lies  in  overcoming  the  attraction  of  the  colloidal  substances 
for  water.  These  are  present  in  the  form  of  hydrosols  as  well  as 

1  Salomon.     Die  stadtische  Abwasserbeseitigvmg  in  Deutschland,   Vol.  II,  p.  21. 

2  Inspected  Dec.  7,  1908. 

3  Inspected  Aug.  27,  1908. 

4  Inspected  Dec.  4,  1908. 

5  1.  c.,  p.  40. 


DRYING  OF  SLUDGE  133 

hydrogels.  According  to  van  Bemmelen1  the  gels  are  mem- 
branes formed  by  precipitation  which  form  a  network  of  amor- 
phous connected  parts  which  are  swelled  up- by  the  liquid  ab- 
sorbed. The  structure  of  such  "micells"  is  described  by  von 
Biitschli2  as  honeycombed.  The  hypothesis  of  a  greatly  en- 
larged surface  would  best  explain  the  peculiarly  stubborn  reten- 
tion of  water  in  fresh  sludge. 

Drying  in  the  air  in  Sedimentation  Tanks. — Difficulties  attend- 
ing the  drying  of  sludge  and  the  great  expectations  entertained 
have  rendered  the.  development  of  different  methods  most 
timely. 

The  most  primitive  method — that  of  letting  it  lie  in  the  tanks 
out  of  use  in  intermittent  operation  has  been  tried  at  the  Konigs- 
berg3  irrigation  fields,  where  the  foul  odors  from  decomposition 
could  do  no  harm.  Two  settling  tanks  used  for  preliminary 
clarification  for  irrigation,  which  were  filled  alternately  with 
sludge,  were  used  there  as  drying  basins,  as  the  farmers  refused 
to  take  the  wet  sludge,  even  when  it  was  given  them.  This 
method  was  soon  seen  to  be  unsuccessful.  As  was  to  be  expected, 
the  sludge  did  not  become  spadable  when  wanted,  so  that  con- 
siderable cost  was  entailed  for  removing  the  wet  material. 

The  same  method  of  drying,  however,  has  been  in  use  for  years 
at  Copenick,4  near  Berlin.  Sludge  procured  by  precipitation 
with  large  amounts  of  lignite  and  an  addition  of  aluminum  sul- 
phate is  not  to  be  compared  with  ordinary  settled  sludge  in  its 
physical  attributes. 

In  Special  Tanks. — At  other  places,  formerly  at  Braunschweig, 
Cassel,  Frankfort  and  still  in  Wimbledon  and  Huddersfield,5 
England,  Essen,  Remscheid  and  Elberfeld-Barmen,  and  recently 
in  different  English  plants,  Birmingham,  e.g.,  special  tanks  are 
used,  partly  with  paved  or  masonry  sides,  partly  with  simple 
earth  embankments.  This  plan  was  abandoned  at  Cassel,  be- 
cause the  sludge  would  not  become  firm,  in  Braunschweig6 
because  of  the  bad  odors.  Dunbar  writes  about  Wimbledon:7 

1  Dr.  Victor  Poschl.    Einfuhrung  in  die  Kolloidschemie,  Dresden.  1908.    Von  Bemmellen. 
Die  Absorption;  Bildung  und  Struktur  des  Gels.  Zeitschrift  f.  anorg.    Chemie,  Vol.  XVIII. 

2  Biitschli.      Untersuchungen    iiber    Mikroskopische    Schaume    und    das    Protoplasms. 
Leipsig,  1892.    See  also,  Zeitschr.  f.  anorg.      Chemie,  Vol.  XXIII,  p.  326. 

-  a Salomon.     Die  Abwasserbeseitigung  in  Deutschland,  Vol.  II,  p.  696. 

4  Inspected  May  9,  1908. 

5  Inspected  May  13,  1909. 

6  Stadtische  Festschrift  fur  die  Teilnehmer  an  der  Versammlung  Deutscher  Naturforscher 
und  Aertze.      Braunschweig,  1898.     Ref.  Salomon,  Vol.  II,  p.  21. 

7  Prof.  Dr.  Dunbar.     Leitfaden  f.  d.  Abwasserreinigungsfrage,  p.  376. 


134  SEWAGE  SLUDGE 

"It  is  difficult  to  separate  the  water  from  the  sludge.  All  efforts 
to  drain  it  have  failed.  If  it  is  allowed  to  stand  in  open  tanks 
with  a  porous  bottom  it  is  often  months  before  it  can  be  removed 
with  spades.  Each  rain  reduces  it  to  its  original  condition.  In 
Wimbledon  after  six  months'  treatment  it  formed  a  thick  liquid 
mass  of  highly  offensive  character  and  77.5  per  cent,  water.  If 
this  mass  of  sludge  is  deposited  in  lagoons,  as  is  often  done  in 
England,  the  whole  neighborhood  suffers  from  the  unbearable 
nuisance." 

In  Essen  sludge  obtained  by  clarifying  with  lime  in  Rothe- 
Rockner  towers  is  placed  in  gigantic  earthen  basins  where  it 
slowly  digests.  The  water  rising  to  the  surface  can  be  drawn 
off  by  some  device.  Odors  are  here  exceptionally  slight,  as  the 
hydrogen  sulphide  is  combined  with  iron.  This  plant  also  is 
soon  to  be  abandoned. 

The  method  was  nearly  the  same  at  Frankfort.  When  it  was 
seen  how  great  the  cost  and  how  bad  the  conditions  were, 
mechanical  means  of  drying  were  resorted  to. 

This  method  of  drying  was  tried  at  Elberfeld-Barmen1  in  spite 
of  the  failures  experienced  elsewhere,  and  huge  tanks  were  built, 
which  had  to  be  increased  in  number  at  the  end  of  a  year,  into 
which  the  fresh  sludge  was  pumped.  As  was  to  be  expected,  the 
sludge  required  a  long  time  to  become  spadable,  and  it  became 
putrid. 

Another  example  is  seen  in  the  wells  of  Mairich.  There  are 
two  such  plants  at  Remscheid.  In  one  the  attempt  is  made  to 
dry  the  sludge  in  tanks  3.3  ft.  (1  m.)  deep,  where  it  is  to  drain 
through  walls  made  of  broken  stone.  Up  to  September,  1908, 
after  it  had  been  in  operation  two  years,  the  sludge  had  not 
become  spadable  in  any  of  the  tanks.  In  the  other  plant  the 
draining  was  to  take  place  through  fascines.  There  it  was  found 
that  the  decomposed  sludge  would  not  drain  through  the  fine 
fascines,  and  simply  ran  through  where  they  were  loosely  woven. 

Spreading  in  Thin  Layers. — Fresh  sludge  dries  more  quickly 
and  without  much  decomposition  where  it  is  spread  out  in  thin 
layers. 

A  method  founded  on  this  idea  has  been  successfully  used 
where  the  necessary  areas  are  available,  and  where  the  odor  causes 
no  nuisance.  The  sludge  is  spread  on  sandy  soil  if  possible.  It 
dries  there,  according  to  its  condition  and  to  the  weather  in  a  few 

1  Inspected  April,  1908. 


DRYING  OF  SLUDGE  135 

days  or  weeks,  and  can  then  be  dug  under  or  carried  off.  This 
can  best  be  done  where,  as  at  Charlottenburg,  the  sludge  obtained 
from  preliminary  clarification  is  spread  on  irrigation  fields. 

In  Germany  it  is  used  at  Mannheim  and  on  several  irrigation 
fields  That  it  is  not  cheap  even  there  is  shown  by  the  fact  that  in 
the  year  1906  clarification  of  the  daily  amount  of  sewage, 
9,246,000  gallons  (35,000  cbm.),  cost  $2530  (10,633  M.),  while  the 
removal  of  92  to  105  cu.  yds.  (70  to  80  cbm.)  sludge  cost  $5,572 
(23,410  M.) — more  than  twice  as  much — although  the  areas  used 
for  drying  were  in  the  immediate  neighborhood  of  the  plant. 

Similar  experiments  have  been  tried  in  recent  years  at  Birming- 
ham, where  the  sludge  was  spread  on  fields  to  dry.  It  was  shown 
that  the  sludge  dried  after  a  while,  but  such  large  areas  were 
necessary  that  it  became  impracticable. 

Burying. — The  so-called  burying  method,  by  which  the  sludge 
is  dried  and  disposed  of  at  the  same  time,  is  much  used  in  Eng- 
land. The  sludge  is  not  spread  upon  the  disposal  areas,  but 
pumped  into  dry  furrows,  where  a  large  percentage  of  the  mois- 
ture is  absorbed  into  the  loose  soil;  as  soon  as  the  sludge  is  firm 
enough  to  support  the  earth  the  furrows  are  filled  in.  If  sludge 
that  has  decomposed  under  water  is  used,  the  same  area  can  be 
used  successively  every  three  years,  according  to  Travis,1  as  is 
done  at  Hampton2  and  Birmingham.3  Where  fresh  sludge  is 
used,  and  especially  where  chemical  precipitation  has  been 
employed,  this  is  not  possible,  as  has  been  demonstrated  at 
Birmingham  and  in  experiments  made  by  the  Emscher  Associa- 
tion, as  the  earth  in  which  it  is  buried  will  not  drain  it.  In 
Birmingham  Dr.  Dunbar4  saw  samples  of  lime  sludge  which  had 
been  buried  more  than  20  years  and  which  retained  the  original 
fecal  odor,  and  was  as  hard  as  blue  clay.  Sludge  which  had  begun 
to  putrify  was  buried,  as  an  experiment,  at  Recklinghausen,  and 
at  the  end  of  a  year  it  had  its  original  odor,  and  was  sticky  and 
slimy,  while  digested  sludge  could  not  be  distinguished  from 
humus. 

Fresn  settled  sludge,  even  when  not  precipitated  with  lime, 
dries  very  slowly  in  such  ditches.  I  convinced  myself  of  this  at 
Birmingham.  Unlimited  areas  were  there  covered  with  such 

1  W.  Oven — Travis.     Some  observations  relating  to  bacterial  tanks  operations.     Trans. 
Soc.  Civil  and  Mechanical  Engrs.     London,  1906. 

2  Inspected  May,  1907. 

3  Inspected  Sept.  15,  1909. 

4  Prof.  Dr.  Dunbar.     Die  Abwasserreinigung  von  Birmingham.     Ges.-Ing.      1908. 


136  SEWAGE  SLUDGE 

sludge-filled  ditches,  and  as  no  satisfactory  results  were  achieved 
it  was  decided  to  return  for  the  present  to  the  method,  already 
mentioned,  of  storing  in  lagoons. 

In  Insterburg1  sludge  8  days  old  is  disposed  of  by  this  method 
of  burying,  but  nothing  has  been  published  as  to  the  results 
obtained.  It  is  doubtful  whether  the  same  ground  can  be  used 
again  for  several  years  when  sludge  that  is  only  partially  decom- 
posed has  been  applied.  If  one  is  not  obliged  to  use  the  same 
area  at  short  intervals,  as  in  the  case  of  small  plants  in  agricul- 
tural regions,  this  method  is  free  from  objection  and  is  to  be 
recommended. 

Composting. — Similar  to  the  method  of  plowing  under  and 
burying,  which  involves  a  rough  mixing  of  the  wet  sludge  with  a 
drier,  porous  substance — the  earth — is  a  method  much  employed 
in  small  plants — i.e.,  mixing  with  street  sweepings,  or  composting. 
This  is  done,  e.g.,  at  Gottingen  and  Cassel  in  Germany.  Where 
these  heaps  of  compost  are  not  quickly  used  by  farmers  they  are, 
of  course,  liable  to  become  a  nuisance. 

Filter-beds. — As  fresh  sludge  may  be  considered  as  solid  mate- 
rial floating  in  a  liquid,  the  next  step  was  to  separate  these  com- 
ponent parts  by  filtration.  This,  too,  has  been  tried  at  different 
places  and  with  various  modifications,  but  without  success,  at 
least  to  the  extent  that  the  sludge,  although  separated  out,  later 
clogged  the  filter  and  decomposed. 

In  Allenstein,2  e.g.,  the  attempt  was  made  to  drain  the  water 
(90  per  cent.)  from  the  sludge  on  carefully  prepared  gravel  filters. 
The  filters  soon  became  clogged,  however,  and  the  sludge  decom- 
posing on  it  produced  such  a  nuisance  by  foul  odors  that  the  plan 
had  to  be  abandoned. 

In  Bielefeld  the  detritus  from  the  preliminary  cleansing  in 
preparation  for  irrigation  is  drained  in  a  similar  manner.  In  the 
report  for  the  year  1905  the  municipal  authorities  state: 

"The  removal  of  the  sludge  settling  out  in  the  clarification 
tanks,  in  which  large  volumes  deposit,  causes  difficulties  which 
should  not  be  underestimated.  In  designing  new  plants  it  is 
strongly  urged  that  drying  beds  of  ample  size  be  provided.  The 
original  number  is  at  present  being  increased  by  two  more.  It  is 
hoped  thus  to  effect  a  speedier  removal  of  the  sludge.  The  water 
drawn  off  from  the  drying  beds  is  again  clarified  and  used  for 

1  Salomon.     Die  stadtische  Abwasserbeseitigung  in  Deutschland,   Vol.  II,  p.  687. 

2  Salomon.      Vol.  II,  p.  17. 


DRYING  OF  SLUDGE  137 

irrigation,  so  that  the  effluent  may  be  carried  to  the  outfall  in  a 
clean  condition." 

In  Leipzig1  a  part  of  the  sludge,  which  is  precipitated  with 
salts  of  iron,  is  led  into  earthen  lagoons.  These  have  drain  pipes 
laid  on  their  level  bottoms,  over  which  is  laid  a  12-in.  (30  cm.) 
layer  of  gravel,  and  on  this  are  laid  tiles  with  close  or  open  joints, 
filled  with  sand.  The  water  from  the  sludge  filters  through  to 
the  drain  pipes  and  is  brought  to  the  plant  for  a  second  clarifica- 
tion. In  summer-the  sludge  often  becomes  spadable  in  2  months; 
in  winter  in  4  or  5  months.  As  the  tanks  are  inadequate  the 
greater  part  of  the  sludge  has  to  be  disposed  of  by  being  conveyed 
to  an  old  river  bed. 

These  examples  show  that  the  mere  filtration  of  fresh  sludge  is 
only  feasible  where  there  are  large  areas  and  ample  time  for  the 
process,  and  that  a  nuisance  from  putrefaction  may  be  expected. 
Means  have  therefore  been  sought  to  reduce  the  time  and  area 
required  for  filtration.  Filter-presses  and  centrifugal  machines 
used  in  certain  chemical  industries  were  recognized  as  such 
means. 

Filter  Presses. — Filter  presses  have  been  employed  for  a  long 
time  at  several  places  as  an  experiment,  e.g.,  at  Cassel  and  Frank- 
fort. The  general  conclusion  is  that  they  are  expensive  to 
maintain  and  operate  and  that  they  possess  little  efficiency.  At 
present  they  are  only  used  in  Germany  with  the  lignite  process, 
e.g.,  in  Potsdam,  Spandau  and  Tegel.  In  Potsdam  it  is  some- 
times impossible  to  press  the  lignite  sludge  dry,  although  it  is 
comparatively  easy  to  separate  it  from  the  water.  According 
to  reports  furnished  me,  8  to  10  per  cent,  of  the  pressed  sludge 
is  too  moist.  This,  even  when  pressed  into  briquettes  for  fuel, 
becomes  soft  again  in  the  rain,  so  that  it  is  necessary  to  store  it 
under  shelter.  Filter  presses  are  no  longer  used  in  Germany  in 
large  plants  for  the  purpose  of  de-watering  fresh  settled  sludge. 

In  England,  on  the  contrary,  filter  presses  are  much  used, 
especially  where  lime  is  employed  as  a  precipitant.  Sludge 
with  lime  is  more  easily  pressed.  Sludge  from  plain  sedimenta- 
tion usually  requires  a  large  addition  of  lime  entailing  a  great 
expense. 

Drum  Filters. — The  dry  process,  described  by  Honig,2  which 

1  Verwaltungsbericht  der  Stadt  Leipzig  und  Mitteilungen  aus  dem   Tiefbauamt.      Plant 
inspected  Dec.  7,  1908,  and  May  10,  1910. 

2  Prof.  M.  Honig.     Ueber  Gewinnung   und  Verwertung  von  staditschem  Klarschlamm. 
Ges.-Ing.,  1910.  p.  26. 


138  SEWAGE  SLUDGE 

has  been  tried  at  Brunn,  is  based  upon  nitration.  The  pressure 
is  produced  by  a  vacuum  under  the  filtering  surface. 

Centrifuges. — More  recently  great  hopes  have  been  placed  in 
centrifugal  machines. 

The  city  of  Frankfort-on-the-Main1  has  conducted  very  ex- 
haustive experiments  with  this  method.  It  may  now  be  seen  in 
operation  on  a  large  scale  at  Harburg,2  Hanover3  and  Frankfort. 

Modern  centrifuges  differ  favorably  from  the  old  filter  presses 
in  that  all  manual  labor  is  avoided,  so  that  the  workmen  need 
no  longer  come  in  contact  with  fresh  sludge  containing  fecal 
matter  or  garbage.  As  yet  little  has  been  published  regarding 
the  cost  and  efficiency  of  this  process.  Reichle  and  Theising/ 
who  have  examined  and  described  the  centrifugal  plant  at  Har- 
burg, give  as  the  cost  of  de-watering  with  centrifuges  62.2  cts. 
per  cubic  yard  (3.42  M.  per  cbm.)  of  dried  sludge,  with  filter 
presses  42  cts.  per  cubic  yard  (2.31  M.  per  cbm.)  under  similar 
conditions.  As  the  cost  was  the  principal  objection  to  the  intro- 
duction of  filter  presses  the  outlook  for  centrifugal  machines  is 
not  very  good.  It  is  still  worse  because  filter  presses  produce  a 
drier  sludge  than  centrifugal  machines,  and  because  the  latter 
produce  a  highly  offensive  liquid  on  account  of  the  large  amount 
of  organic  matter  contained  (according  to  the  above  authorities 
3.7  per  cent,  dried  matter  of  which  91  per  cent,  is  organic). 
It  is  not  to  be  compared  with  the  liquid  from  filter  pressing,  as 
can  be  seen  at  Harburg,  Hanover  and  Frankfort.  The  latter  is 
usually  a  transparent,  pale  yellow  liquid,  as  is  seen  at  Halifax, 
England,5  while  the  effluent  from  centrifugal  machines  is  a  black, 
watery  sludge. 

In  spite  of  these  drawbacks  centrifugal  machines  must  be 
employed  where  the  agricultural  utilization  of  sludge  is  im- 
practicable especially  where  there  is  no  cheap  land  available 
for  storing  the  wet  sludge.  Frankfort-on-the-Main  has  already 
chosen  this  method. 

Drying  by  Heat. — Three  years  ago  an  attempt  was  made  at 
Potsdam  to  dry  sludge  by  heat.  The  lignite  sludge  obtained 
there,  which  was  first  dried  in  filter  presses,  and  which  it  was 

1  Zentrifugen.     Schafer-ter  Meer  System 

2  Inspected  May  7,  1908. 

3  Inspected  Nov.  18,  1908. 

4  Bauinspektor  Reichle  und  Prof.  Dr.  Thiesing.     Mitteilungen  a.  d.  Kgl.  Pruf.-Anst.  f. 
Wassersorgung,  etc.      Vol.  X. 

5  Inspected  Sept.  13,  1909. 


DRYING  OF  SLUDGE  139 

intended  to  used  as  fuel  on  account  of  the  latent  energy  in  the 
lignite,  was  manipulated  experimentally  in  rotary  ovens  with 
fuel  gas.  Similar  attempts  are  now  being  made  at  Frankfort  for 
the  further  drying  of  centrifuged  sludge  containing  about  70 
per  cent,  moisture. 

Electro-osmose. — Experiments  have  also  been  made  at  Frank- 
fort with  the  electro-osmose  process  of  Count  Schwerin,  to  de- 
water  more  easily  slimy  sludge  which,  as  already  mentioned,  is 
difficult  to  dry. 

Dr.  Tillmans1  writes  of  this  method  that  it  prevents  the 
colloidal  condition  of  the  sludge  liquor  for  a  while,  but  that  this 
appears  later.  The  current  used  is  great  but  not  prohibitive. 

More  extensive  experiments  were  to  have  been  made  at  Frank- 
fort, but  the  results  have  not  yet  been  published. 

Results  of  the  Methods  of  Drying  Already  in  Use. — As  regards 
the  results  obtained  the  methods  of  drying  of  fresh  sludge  can  be 
divided  into  two  groups:  those  in  which  there  is  a  steady  drying 
but  which  are  expensive  to  install  and  operate  (filter  presses  and 
centrifugal  machines)  and  those  which  cost  less  but  give  no 
assurance  of  effecting  the  desired  result  (draining,  irrigation 
and  burying). 

The  problem  of  rational  drying  has  not  yet  been  solved. 
The  septic  process  gives  a  better  prospect  of  success. 

i  Dr.  J.  Tillmans,  Zeitschr.  f.  d.  Unters.  d.  Nahrungs.  u.  Genussmittel.    Vol.  XIV  (1907), 
Parts  1  and  2 


CHAPTER  III 
DRYING  SEPTIC  TANK  SLUDGE 

Source. — The  apparatus  for  septic  treatment  differs  from  that 
for  plain  sedimentation  by  its  greater  size. 

Septic  Tank  Method. — The  time  of  flow  through  septic  tanks 
is  usually  from  12  to  24  hours,  during  which  it  decomposes  to  a 
greater  or  less  extent,  according  to  its  composition.  The  sludge 
is  stored  under  water  as  long  as  possible.  The  capacity  is 
generally  arranged  so  that  it  can  remain  from  6  to  12  months. 
The  easily  disintegrated  portions  are  removed  by  digestion. 
The  gases  which  develop  bring  up  particles  of  sludge  which  form 
a  floating  cover.  The  removal  of  sludge  is  performed  in  the 
same  manner  as  with  tanks  for  plain  sedimentation. 

Disadvantages  of  the  Effluent. — The  original  aim  in  the  septic 
treatment  was  to  clarify  the  sewage  more  thoroughly  than  was 
possible  by  plain  sedimentation.  To-day  it  is  known1  that  the 
storage  of  the  large  quantities  of  sewage  from  a  city  until  .it  is 
entirely  decomposed  is  practically  impossible.  It  is  still  believed 
by  many  that  permitting  the  sewage  to  become  partially  septic 
will  produce  as  good  a  biological  clarification  as  when  treated 
in  a  fresh  condition.  That  is  not  the  case;  the  anaerobic  bacteria 
are  favored  and  the  process  of  reduction  initiated  by  the  cus- 
tomary 12  to  24  hours'  storage  in  septic  tanks.  The  reduction 
is  indicated  by  the  formation  of  NH3,  H2S  and  CH4,  and  the 
decrease  of  N2O5  and  N2O3.  With  the  biological  purification 
which  follows,  whether  by  irrigation,  intermittent  sand  filtration, 
contact  beds  or  the  self-purification  that  takes  place  in  a  stream, 
active  aerobic  organisms  are  required,  and  it  proceeds  as  a  process 
of  oxidation.  The  activity  of  the  anaerobes  must  first  be 
checked  and  the  products  of  decomposition  in  the  water  must 
again  be  oxidized.  Purification  is  therefore  retarded  by  the 
septic  process.  Liibbert  says  of  this:2  "A  medium  is  introduced 
into  contact  beds  with  the  septic  sewage  which  offers  conditions 

1  Prof.  Dr.  Dunbar.      Leitfaden  fur  die  Abwasserreinigungsfrage,  pp.  127  and  140. 

Dr.  Liibbert.  Einfuhrung  in  die  Frage  der  Abwasserreinigung.  Zeitschx.  d.  Vereins 
Deutscher-Ing. ,  1909,  Nos.  1-4. 

Dr.  Liibbert.     Die  Abwasserreningung  im  Kleinbetrieb.  Ges.-Ing.,  1909,  p.  265 

2  Dr.  Ltibbert,  1.  c. 

140 


DRYING  OF  SLUDGE  141 

diametrically  opposed  to  those  desired,  and  the  work  of  the 
oxidizing  micro-organisms  is  rendered  more  difficult  if  they-  are 
to  conquer  in  a  struggle  in  which  their  antagonists,  the  ferments 
and  decomposing  agencies,  have  the  upper  hand." 

Another  unpleasant  result  of  the  septic  method  is  often  the 
unsatisfactory  sedimentation  which  takes  place  in  the  tianks. 
The  gases  rising  from  the  sludge  at  the  bottom  interfere  with  the 
settling  action  and  bring  up  flakes  of  sludge.  Particles  fall 
from  the  scum.  In  spite  of  all  precautions,  such  as  scum 
boards  and  the  arrangement  of  the  outlet  openings  far  below  the 
water  surface,  the  effluent  is  not  as  clear  as  in  plain  sedimen- 
tation plants;  it  is  thus  sometimes  necessary,  as  at  Birmingham, 
to  insert  sedimentation  tanks  and  roughing  filters  between 
septic  tanks  and  contact  beds. 

Further,  the  septic  effluent  usually  contains  much  sulphu- 
reted  hydrogen  which  is  dispersed  in  the  air  by  the  subsequent 
distribution  of  the  sewage  on  contact  beds,  especially  if  this  is 
done  with  a  sprinkler  nozzle  or  revolving  sprinklers,  producing 
much  worse  odors  than  the  worst  sludge. 

Sewage  is,  therefore,  detrimentally  affected  by  the  septic 
treatment.  In  handling  the  sludge,  however,  decomposition 
offers  distinct  advantages. 

Advantages  for  the  Sludge. — Sludge  which  lies  submerged  for 
months  at  a  temperature  not  too  low,  undergoes  a  profound 
alteration.  Its  organic  constituents  are  attacked  by  putre- 
faction and  the  products  of  decomposition  partly  disappear  in 
the  form  of  gas.  There  has  been  much  discussion  concerning 
the  amount  of  sludge  consumption  and  a  large  amount  of  liter- 
ature exists  on  the  subject,  but  the  question  of  amount  does 
not  touch  the  kernel  of  the  matter.  The  quantity  of  organic 
material  destroyed  is  much  less  important  than  whether  the 
sludge  acquires  desired  qualities  by  septic  treatment. 

Characteristics  of  Septic  Sludge. — Sludge  treated  in  efficient 
septic  tanks  differs  from  fresh  sludge  by  its  color,  which  is  usually 
very  black  on  account  of  the  iron  sulphide  contained,  by  its 
less  disagreeable  odor,  by  its  greater  concentration — it  contains 
20  per  cent,  of  dried  matter  as  compared  with  5  to  10  per  cent, 
in  fresh  sludge — and  by  the  ease  with  which  it  drains.  These 
last  differences  are  most  important 'in  treatment.  They  arise 
from  the  destruction  of  the  water-binding  colloidal  substances. 
They  are  of  the  utmost  importance  in  drying. 


142  SEWAGE  SLUDGE 

Drying  Septic  Sludge. — Drying  by  burying,  which  is  very 
difficult  with  fresh  sludge,  is  easily  accomplished  with  septic 
sludge.  This  method  has  been  used  in  Hampton  for  the  past 
5  years.  In  Birmingham  there  were  no  difficulties  with  burying 
so  long  as  septic  sludge  was  produced.  Drying  on  porous  areas 
is  more  easily  carried  on  than  with  fresh  sludge,  because  it 
drains  so  readily.  In  Unna1  it  became  nearly  spadable  in  the 
comparatively  short  time  of  from  4  to  6  weeks.  In  Miilheim- 
Ruhr,2  where  it  is  piled  3.3  ft.  (1  m.)  high,  it  takes  from  8  to  12 
weeks  in  summer  to  become  spadable.  Fresh  sludge  requires 
as  least  one  year.  Septic  sludge,  on  the  other  hand,  is  not  so 
well  adapted  to  treatment  in  filter  presses3  or  centrifugal  ma- 
chines.4 This  is  of  little  consequence,  however,  as  these  costly 
devices  are  only  resorted  to  when  cheaper  methods,  such  as 
draining  the  fresh  sludge,  fail. 

Decline  of  the  Septic  Method. — Although  the  advantages  of 
septic  treatment  have  been  known  for  years  and  been  uniformly 
confirmed,  it  is  steadily  declining  on  account  of  its  disadvantages 
and  the  cost  of  the  plant 

The  construction  and  maintenance  of  a  septic  treatment  plant 
are  expensive,  because  the  capacity  required  for  the  storage  of 
the  sewage  must  be  taken  at  from  6  to  12  times  that  for  plain 
sedimentation — which  is  usually  2  to  4  hours. 

Large  plants  are  constantly  being  converted  from  the  septic 
tank  process  to  plain  sedimentation.  In  Manchester,  e.  g.}  whole 
rows  of  septic  tanks — one-third  of  the  entire  installation — have 
been  changed  to  sedimentation  tanks,  and  in  Birmingham,  also, 
which  has  the  largest  plant  in  the  world,  the  greater  number  of 
the  former  septic  tanks  have  been  changed  to  tanks  for  plain 
sedimentation,  in  spite  of  the  objectionable  characteristics  of 
fresh  sludge. 

Former  advocates  of  the  septic  principle  are  now  constructing 
sedimentation  plants,  e.g.,  Travis  at  Norwich5  in  which  not  only 
is  the  sewage  to  remain  fresh,  but  by  which  the  sludge  is  to  be 
removed  at  short  intervals,  as  in  the  process  of  plain  sedimenta- 
tion. 

1  Inspected  May  15,  1908. 

2  Inspected  May  15,  1909. 

3  Royal  Com.  on  Sew.  Disp.  5th  Rep.  London,  1908. 

4  Nach  Angaben  der  Hannoverschen  Maschinenbau-A.-G.  vorm.  Georg  Egestorff  (Zen- 
trifugen  System  Schafer-ter  Meer)  und  Mitteilungen  aus  d.  Kgl.  Pruf    Anst.  f.  Wasserv. 
u.  Abwasserbeseit.      Vol.  X,  p.  192. 

5  Inspected  Sept.  17,  1909.     Lit:  Surveyor,  1908.     Nos.  855  and  856,  p.  672. 


CHAPTER  IV 
THE   DRYING   OF   EMSCHER  TANK  SLUDGE 

Emscher  Tanks. — Midway  between  the  sedimentation  and 
septic  processes  of  sewage  purification  and  sludge  treatment  is 
the  method  exemplified  by  the  Emscher  tank  The  details  of 
construction  are  made  clear  by  various  illustrations.1  In  the 
Emscher  tank  clarification  takes  place  in  a  chamber  from  which 
the  sludge  is  drawn  off  continuously  and  automatically.  The 
liquid  passes  through  the  chamber  in  from  one  to  two  hours. 
Because  of  this  brief  period  of  clarification  and  the  immediate 
removal  of  the  sludge,  putrefaction  of  the  liquid  occurs  less  than 
is  often  the  case  in  treatment  by  sedimentation. 

Action  in  the  Sludge  Chamber. — The  sludge  flows  from  the 
sedimentation  chamber  into  a  well-shaped  compartment  below, 
in  which  it  remains,  on  an  average,  two  or  three  months. 

The  processes  which  go  on  in  this  sludge  chamber,  so  far  as 
they  have  been  ascertained,  are  essentially  different  from  the 
putrefaction  in  ordinary  septic  tanks  with  currents  passing 
through  them;  for  the  gases,  escaping  in  large  quantities,  unlike 
those  in  septic  tanks,  contain  very  little  hydrogen  sulphide.  They 
consist  mostly  of  methane  and  carbonic  acid.  The  apparent 
cause  of  this  phenomenon  is  the  fact  that  the  liquid  covering  and 
surrounding  the  sludge  is  renewed  to  but  a  very  slight  extent. 
It  becomes,  accordingly,  thoroughly  septic  very  quickly.  The 
albuminous  matters  that  have  been  set  free  in  it  are  decomposed 
and  can  form  no  more  hydrogen  sulphide.  In  septic  treatment, 
on  the  other  hand,  fresh  sewage  is  continually  brought  into 
contact  with  the  putrefying  sludge  and  decomposes,  so  that  the 
suspended  as  well  as  the  dissolved  albuminous  matters  are  con- 
tinually being  acted  upon,  developing  hydrogen  sulphide. 

From  the  sludge  itself  either  very  little  hydrogen  sulphide  is 
developed,  or  else  it  decomposes,  or  the  sulphur  is  otherwise  com- 
bined. Investigations  of  this  question  are  now  being  taken  up. 

Removal  of  Sludge. — The   sludge  is   drawn  off  through  iron 

1  1.   Dr.  Ing.  Imhoff,  D.  R.  P.  No   187,723,  Kl.  85.  c. 

2.  Regierungsbaumeister    Helbing.     Die    Durchfiihrung    des    Emschergenossenschaft 
gesetzes.     Tech.  Gem.,  Vol.  X,  No.  13. 

3.  Baurat   Middeldorf.      Die    Arbeiten    der     Emschergenossenschaft.      Deutsche    Bau- 
zeitung,  1909.     Nos.  78,  79,  81. 

4.  Dr.  Ing.  Imhoff,  A  New  Method  of  Treating  Sewage.     Surveyor,  1909.     No.  905. 

143 


144 


SEWAGE  SLUDGE 


pipes  which  reach  to  the  bottom  of  the  tanks  and  lead  out 
through  the  side  of  the  tank  about  3  1/3  ft.  (1  m.)  below  the  sur- 
face. When  the  valve  closing  the  end  of  the  pipe  projecting  into 
the  tank  is  opened,  the  sludge  is  forced  out  by  the  weight  of  the 
liquid.  This  does  not  interrupt  the  process  of  clarification. 

Character  of  the  Sludge. — Experience  has  so  far  indicated  that 
sludge  from  Emscher  tanks  exhibits  in  a  higher  degree  all  the 
favorable  properties  of  that  from  septic  tanks. 

Amount  of  Moisture. — The  water  contained  is  decidedly  lower 
in  amount  than  that  in  sludge  from  septic  tanks,  although  the 
Emscher  sludge  is  always  drawn  off  under  water.  In  10  analyses 
of  the  digested  sludge  from  the  Recklinghausen-Ost  clarification 
plant  (Table  I)  I  found  an  average  of  79.34  per  cent,  moisture; 
in  16  analyses  from  the  Essen-Nordwest  plant  (Table  II)  an 
average  of  75.6  per  cent.,  and  in  6  analyses  from  the  Bochum 
plant  (Table  III)  an  average  of  75.88  per  cent.  Each  of  these 
samples  was  an  average  sample  from  a  larger  amount  of  sludge 
(52.3-123.0  cu.  yds. -40-94  cu.  m.).  The  greatest  amount 
of  moisture  found  at  Essen-Nordwest  was  81.8  per  cent.  (Oct.  11, 
1909,  when  there  was  only  a  thin  layer  of  sludge  in  the  tank). 
The  least  was  71.35  per  cent.  (May  8,  1909).  Of  the  16  average 
samples  taken,  9  contained  less  than  75  per  cent,  moisture. 

TABLE  I  (Condensed) 

RECKLINGHAUSEN  CLARIFICATION  PLANT.     (6  EMSCHER  TANKS) 
Summary  of  Analyses  of  Wet  Decomposed  Sludge  from  samples  taken 
from  June  14,  1907,  to  July  26,  1909. 


Per  cent. 

Ave. 

Max. 

Min. 

Wet  sludge 
Moisture  contained  
Dried  material 

79.34 
20  66 

84.2 
25  0 

75.0 
15.8 

Dried  material 
Mineral  matter  
Organic  matter  
Nitrogen  
Fats1.    .                             

54.8 
45.2 
1.56 
6.41 

64.4 
55.9 
1.71 
6.75 

44.1 
35.6 
1.47 
6.07 

1  Analyses  made  on  two  days  only. 


DRYING  OF  SLUDGE  145 

TABLE  II  (Condensed) 
EssEN-N.  W.  CLARIFICATION  PLANT  (9  EMSCHER  TANKS) 

Summary  of  analyses  of  Wet  Decomposed  Sludge  from  samples  taken 
from  Apr.  8,  1909,  to  Oct.  11,  1909. 


Per  cent. 

' 

Ave. 

Max. 

Min. 

Wet  sludge 
Moisture  contained  

75.6 

81.8 

71.35 

Dried  material  

24.4 

28.65 

18.2 

Dried  material 

Mineral  matter  

45.08 

53.51 

37.6 

Organic  matter  
Nitrogen  
Fats1.  . 

54.92 
1.22 

4.89 

62.4 
1.43 
7.36 

46.49 
1.015 
3.44 

1  Analyses  made  on  seven  days  only. 


TABLE  III  (Condensed) 
BOCHUM  CLARIFICATION  PLANT  (18  EMSCHER  TANKS) 

Summary   of  analyses  of  Wet  and   Decomposed  Sludge  from  samples 
taken  from  Feb.  11,  1909,  to  Aug.  13,  1909. 


Per  cent. 

Ave. 

Max. 

Min. 

Wet  sludge 

Moisture  contained  

75.88 

79.71 

72.97 

Dried  material  

24.12 

27.03 

20.29 

Dried  material 

Mineral  matter  

59.49 

63.98 

49.3 

Organic  matter 

40.51 

50.7 

36.02 

Nitrogen  

1.102 

1.46 

0.87 

Fats1  ;  

8.73 

12.3 

5.82 

Analyses  made  on  four  days  only. 
10 


146  SEWAGE  SLUDGE 

ANALYSES  OF  WET  AND  DECOMPOSED  SLUDGE 


Clarification  Plant  at 

Workman's  Mine  "Schwerin" 
near  Rauxel  i.  W. 

Beckum  i.  W. 

Number  of  tanks 

2 
Nov.  6,  1909 
Per  cent. 
80.4 
19.6 

49.0 
51.0 

Dec.  1,  1909 
Per  cent. 
77.6 
22.4 

64.0 
36.0 
1.34 
2.61 

Date  .  .  .  
Wet  sludge  
Moisture  contained 

Dried  material  
Dried  material 
Mineral  matter  
Organic  matter  
Nitrogen                         .... 

Fats  



With  70  per  cent,  moisture  Emscher  sludge  is  still  like  gruel  in 
consistency.  It  is  completely  mobile,  flowing  of  itself  in  slightly 
inclined  channels.  It  can  also  be  raised  from  the  bottom  of  the 
tank  by  means  of  an  ordinary  trench  (membrane  or  diaphragm) 
pump.  Fresh  sludge  of  the  same  degree  of  concentration  (70  per 
cent,  moisture)  is  generally  rather  firm. 

This  peculiar  difference  is  in  part  due  to  the  fact  that  the  fibrous 
or  clogging  constituents  are  almost  entirely  destroyed,  and  in  part 
to  the  microscopic  gas  bubbles  which  take  the  place  of  the  water 
between  the  separate  particles  of  solid  matter  which,  being  sur- 
rounded by  minute  fluid  films,  renders  the  entire  mass  mobile. 

Odor. — The  odor  of  wet  Emscher  sludge  can  only  be  detected 
near  by,  and  is  only  noticeable  when  it  has  been  warmed  to  158 
or  176°  F.  (70°  or  80°  C.).  It  smells  like  rubber,  or  sometimes 
like  tar  or  peptone.  No  disagreeable  odors  can  be  perceived  a 
few  feet  away,  even  when  the  sludge  is  being  drawn  off 

Analytical  data  of  the  organic  material,  fats  and  nitrogen  con- 
tained can  be  found  in  Tables  I  to  III. 

Drying. — Only  methods  based  upon  drainage  need  be  con- 
sidered with  reference  to  the  drying  of  sludge  from  Emscher 
tanks,  according  to  the  following  experiments,  as  they  show  that 
these  methods  are  the  simplest  and  cheapest  besides  being  par- 
ticularly adapted  to  Emscher  tank  sludge. 

EXPERIMENTS  WITH  DRAINING 

General  Remarks. — The  question  of  the  facility  with  which 
sludge  which  has  decomposed  under  water  may  be  drained  and 


DRYING  OF  SLUDGE  147 

the  reasons  underlying  this  have  not  yet  been  thoroughly  in- 
vestigated. At  the  experimental  plant  of  the  Emscher  Associa- 
tion at  Essen- Ruhr  in  1907,  it  was  shown  that  the  sludge  received 
there,  when  placed  on  porous  material  to  a  depth  of  about  10  in. 
(25  cm.)  frequently  became  spadable  in  8  to  10  days. 

The  tank  from  which  the  sludge  came  was  built  in  1906  by 
Baurat  Middeldorf  and  Engineer  Wattenberg  after  an  English 
model — that  of  Travis  at  Hampton1  (Middlesex).  At  the 
suggestion  of  Dr.  Imhoff  it  was  altered  before  being  put  into 
service  so  that  the  chamber  which  received  the  sludge  and  where 
it  was  to  decompose  would,  in  distinction  from  the  Travis  tank, 
have  no  current  of  sewage  passing  through  it.  This  was,  there- 
fore, the  first  application  of  the  Emscher  tank  treatment.  It 
differed  in  outward  form  from  the  true  Emscher  tank  in  being 
more  .shallow.  This  has  an  unfavorable  influence  on  the  con- 
tained moisture  and  the  time  required  for  drying. 

The  amount  of  water  contained  is  an  important  factor  in  the 
drying  of  sludge.  The  less  water  it  contains  the  more  quickly  it 
dries.  Fresh  sludge  often  contains  over  90  per  cent,  of  water, 
sometimes  95  to  97  per  cent.  That  obtained  from  the  shallow 
tanks  of  the  experimental  plant  had  an  average  of  less  than  90 
per  cent.  It  is  therefore  possible  that  the  rapid  drying  was  a 
result  of  the  small  amount  of  moisture  contained  and  was  due  to 
evaporation  rather  than  to  drainage.  As  no  large  amount  of 
drainage  water  was  observed,  it  was  inferred  that  draining  played 
no  important  part  in  the  process  of  drying.  After  altering  the 
drying  beds  I  succeeded  in  securing  and  measuring  large  amounts 
of  drainage  water. 

EXPERIMENT  I 

The  Drying  Bed. — The  drying  bed  for  the  experimental  plant 
is  65.6  ft.  (20  m.)  long  and  16.4  ft.  (5  m.)  wide.  The  bottom  is  of 
fairly  impervious  clay  and  the  walls  of  masonry  in  cement 
mortar.  It  is  filled  to  about  20  in.  (1/2  m.)  in  depth  with  coarse 
boiler  clinker,  over  which  a  layer  of  fine  clinker  6  in.  (15  cm.) 
thick  is  spread.  Drain  pipes  connecting  at  right  angles  with  a 
collecting  drain  serve  to  draw  off  the  wnter.  It  is  divided  by 
planks  into  5  compartments  of  215  sq.  ft.  (20  qm.)  each. 

Before  the  experiment  was  begun  I  altered  the  filling  material 

1  Dr.  Travis,  Hampton.     Surveyor,  1905.     No.  703. 


148 


SEWAGE  SLUDGE 


and  provided  the  basin  with  diagonal  drainage  in  place  of  the 
original  drain. 

Carrying  Out  the  Experiment. — July  8,  1907,  31.4  cu.  yds.  (24 
cbm.)  of  digested  sludge  from  the  experimental  tank  was  placed 
on  the  drying  bed  thus  prepared.  The  specific  gravity  of  the  wet 
sludge  was  1.033.  27.8  tons  (24.8  long  tons)  were  consequently 
delivered.  The  depth  was  about  9  1/2  in.  (24  cm.).  Analysis  of 
an  average  sample  of  the  wet  sludge  showed  92.34  per  cent, 
moisture.  Of  the  27.8  tons  (24.8  long  tons),  therefore,  25.6  tons, 
(22.9  long  tons)  were  water.  The  sludge  became  spadable  in  8 
days.  During  this  time  1833  gallons  (6.94  cbm.)  of  drainage 
water  was  obtained  =  30. 16  per  cent,  of  the  original  amount  of 
water.  The  firm  sludge  examined  a  few  days  later  contained 
65.4  per  cent,  moisture.  The  amount  was  then  found  to  be  4.14 
tons  (3.7  long  tons).  It  still  contained  2.69  tons  (2.4  long  tons) 


130 


60 


40  * 


FIG.  31. — Drainage  of  sludge  at  the  experimental  plant  July  8  to  July  17,  1907. 


of  water.  The  loss  of  water  was,  therefore,  22.96  tons  (20.5 
long  tons).  Of  this,  7.77  tons  (6.94  long  tons),  or  33.85  per  cent., 
ran  off  as  drainage  water. 

The  graphic  representation  of  the  results  of  the  measurements 
made  given  in  Fig.  31  shows  that  the  amount  running  off,  begin- 
ning with  26.7  gallons  (101  1.)  per  hour,  rose  in  the  course  of  the 
first  day  to  34.3  gallons  (130  1.)  per  hour,  and  then  gradually 
diminished,  and  that  the  drainage  was  practically  completed  in 
5  days.  Measurements  taken  July  13  showed  but  2.4  gallons 
(1  9.)  per  hour.  Observations  were  continued  until  July  17. 
The  last  measurements  showed  0.53  gallon  (2.1)  per  hour. 


DRYING  OF  SLUDGE  149 

Results. — Drainage  was  shown  by  this  to  play  an  important 
part  in  drying  sludge.  More  than  one-third  of  the  water  lost 
reached  the  place  of  measuring  as  drainage  water.  Draining  was 
practically  completed  in  the  first  half  of  the  8  days  taken  for 
drying. 

The  large,  open  drying  bed  was  not  adapted  to  the  precise 
determination  of  the  relation  of  draining  to  evaporation,  as  it 
was  neither  protected  from  rain,  nor  had  it  an  absolutely  imper- 
vious foundation;  and  as  the  amount  of  drainage  water  could  not 
be  directly  measured,  but  had  to  be  estimated  from  measure- 
ments taken  several  times  daily,  I  therefore  made  experiments  on 
a  smaller  scale  with  an  apparatus  which  represented  a  fraction  of 
an  ideal  drying  bed. 

Apparatus. — In  a  water-tight  glass  box  holding  2.5  cu.  ft. 
(71.7  1.)  and  provided  with  a  faucet,  was  constructed  a  "filter- 
frame,"  consisting  of  slag,  and  resting  on  a  grating;  i.e.,  pieces  of 
slag  were  laid  in  5  layers,  each  layer  composed  of  finer  fragments 
than  the  one  below,  so  that  the  finer  material  would  not  fall 
through  the  interstices  of  the  coarser  layer  below.  The  top  layer 
was  composed  of  washed  Rhine  sand  0.02  to  0.01  in.  (1/2  to  1/4 
mm.)  in  size.  A  wooden  frame  holding  1.05  cu.  ft.  (30 1.)  covered 
with  zinc,  pressed  firmly  into  the  top  layer,  served  to  hold  the 
sludge. 

The  experiments  were  conducted  in  a  shed,  protected  from  the 
rain. 


DRAINAGE  EXPERIMENT  II 

On  the  18th  of  July,  1907,  52.8  Ibs.  (24  kg.)  of  decomposed 
sludge  from  the  experiment  station  were  placed  in  the  frame  of 
the  filter.  The  first  of  the  drainage  water  appeared  in  11/2 
hours. 

The  whole  amount  of  the  water  drained  off  was  retained  and 
measured  every  24  hours.  The  results  are  given  in  Table  IV, 
the  first  column  showing  the  daily  volume,  and  the  second 
column  the  sum  of  the  volumes  up  to  the  day  given,  the  3rd 
and  4th  the  same  quantities  in  per  cent,  of  the  original  amount 
of  water  contained  in  the  sludge. 


150 


SEWAGE  SLUDGE 
TABLE  IV.  (Condensed) 


Date 

Effluent 

Percentage  of  the  total 
amount  of 

For  24 

hours 

Total 

47.9721bs. 

21.756kg. 

Gallons 

Cubic 
Centi- 
meters 

Gallons 

Cubic 
Centi- 
meters 

For  24  hours 

Total 

July  19  
July  20  
July  24  
Aug  1 

.511 
.524 
.164 
.004 
.066 

1930 
1980 
620 
15 
250 

.511 
1.035 
2.264 
2.719 

:     2  .  785 

1930 
3910 
8550 
10269 
10519 

8.87 
9.1 
2.85 
0.07 
1.25 

8.87 
17.97 
39.15 
47.06 
48.31 

Aug.  2-7  

The  sludge  became  spadable  in  about  8  days.  The  course  of 
drying  was  followed  by  the  analysis  of  samples  taken  at  intervals. 
A  sample  of  the  same  sludge  used  in  the  experiment  was  pre- 
served in  an  open  water-tight  tub  and  analyzed  at  the  end  of  the 
period. 

The  results  of  the  analyses  at  the  beginning  and  end  as  well 
as  the  two  between  are  given  in  the  following  table : 

TABLE  V. 


Date 

Water. 
Per  cent. 

Dried  material.     Per  cent. 

Total 

Loss  on  ignition 

Mineral  residue 

July  18 

90.62 
79.60 
69.19 
56.20 

9.38 
20.40 
30.81 
43.80 

40.52 
37.20 
35.50 
33.10 

59.48 
62.80 
64.50 
66.90 

July  24    

Aug.  1  
Aug.  7  

The  sample  left  in  the  open  tub  showed  the  following  composition: 


July  24  

90.05 

9.95 

39.46 

60.54 

The  drained  sludge  had  been  reduced  in  moisture  from  90.62 
per  cent,  to  56.20  per  cent.  In  6  days  (July  24)  it  had  been 
reduced  to  79.6  per  cent.,  while  that  in  the  tub,  which  lost  only 
by  evaporation,  contained,  after  the  same  time  and  with  equal 
depth,  90.05  per  cent. 


DRYING  OF  SLUDGE 


151 


Drainage  Water. — The  results  of  an  analysis  of  the  drainage 
water  are  as  follows 


TABLE  VI. 

EXAMINATION   OF    WATER  DRAINED   OFF   FROM   JULY   27  TO  AUGUST    1, 
CLEAR,  COLORLESS,  ODOR  SLIGHTLY  EARTHY. 


Parts  per  million 
=mg.  per  1. 

2103 

Residue  from  ignition  

1601 
502 

76  .  2  per  cent. 
23.8  per  cent. 

Oxygen  consumed  (according  to  Kubel)  
Total  nitrogen  

221.6 
99.4 

KMnO4  consumed. 

Of  which:  Organic  nitrogen  
Nitrogen  in  ammonia  

2.8 
78.4 

Nitrogen    in    nitrates    and    nitrites 
fXr-iO-.  anrl       NToO-,1    . 

18.2 

When  kept  10  days  in  a  closed  bottle  at  the  temperature  of  the 
room  no  sulphuretted  hydrogen  was  found. 

Results. — The  water  originally  contained  was,  according  to  the 
analysis,  5.747  gallons  (21.7561).  The  total  volume  of  water 
removed  can  only  be  estimated  in  a  round-about  way  as  the 
samples  taken  for  analysis  during  the  test  reduced  the  amount 
of  sludge  present.  Without  material  error  we  may  consider  the 
amount  of  mineral  matter  2.945  Ibs.  (1.336  kg.)  unchanged  and 
find,  then,  as  the  final  weight  of  the  drained  sludge  7.54  Ibs. 
(3.42  kg.)  and  of  the  moisture  contained  4.240  Ibs.  (1.923  kg.). 

We  have,  then: 

Reduction  of  sludge     85.76  per  cent. 

Reduction  of  water     91.25  per  cent. 

There  were  drained  off  2.682  gallons  (10.1591). 

=  48.4  per  cent,  of  the  original  volume  of  water. 
=  53.1  per  cent,  of  the  water  removed. 

According  to  this  there  were  evaporated  20.537  Ibs.  (9.314  kg.) 
of  water: 


=  42.8  per  cent,  of  the  original  volume  of  water. 
=  46.9  per  cent,  of  the  water  removed. 


152  SEWAGE  SLUDGE 

This  experiment  therefore  confirmed  the  results  of  the  first 
experiment  and,  moreover,  showed  that  the  decomposed  sludge 
of  the  experimental  plant  loses  under  favorable  conditions  for 
draining,  more  than  half  of  the  water  removed  in  this  way. 
A  further  disintegration  of  the  organic  matter  goes  on  during 
the  drying.  Odors  are  not  perceptible  (if  the  sludge  is  stored 
up  in  the  open  air  without  draining;  decomposition  is  not  nearly 
so  energetic). 

An  analysis  of  the  drainage  water  shows  indications  of  bio- 
logical purification  (putrescibility,  nitrates  and  nitrites)  although 
the  importance  of  this  is  lessened  by  the  fact  that  the  inference 
was  drawn  merely  from  a  sample  taken  at  the  end  of  the  experi- 
ment. 

COMPARATIVE    EXPERIMENTS    WITH   FRESH    AND    DECOMPOSED 

SLUDGE 

These  experiments  were  made  to  ascertain  whether  there  is 
any  difference  in  the  facility  of  drainage  between  fresh  and 
decomposed  sludge,  and  whether  the  superiority  of  our  de- 
composed sludge  in  this  respect  is  not  due  to  the  fact  that  the 
fresh  sludge  itself  is  exceptionally  capable  of  being  drained. 

Top  Layer  of  the  Draining  Bed. — In  the  following  experiments 
coal  waste  was  used  as  a  top  layer  instead  of  pulverized  slag, 
a  material  that  was  cheap  to  procure  in  the  Emscher  district. 
92  per  cent,  of  this  covering  material  was  composed  of  grains  from 
2  to  4  mm.  in  size,  8  per  cent,  from  1  to  2  mm. 

This  combustible  material  was  used  because  of  the  intention 
to  burn  the  sludge  in  some  part  of  the  Emscher  district.  In 
shoveling  off  the  spadable  sludge  a  portion  of  the  top  layer  of 
the  drying  bed  adheres  to  the  sludge.  If  this  top  layer  is  com- 
bustible the  calorific  value  of  the  sludge  is  increased;  if  not,  it  is 
reduced. 

Porosity  of  the  Draining  Beds. — The  draining  beds  were  ex- 
amined as  to  their  porosity  before  the  experiments  were  begun. 
Equal  volumes  of  a  preliminary  dose  of  water  were  evenly  flowed 
over  the  surfaces  of  the  beds  and  the  time  and  amount  flowing 
off  were  noted.  The  porosity  proved  to  be  practically  uniform 
as  was  to  be  expected  with  the  similar  construction  of  the  beds. 
The  beds  were  then  allowed  to  dry  for  several  days. 


DRYING  OF  SLUDGE 


153 


COMPARATIVE  EXPERIMENTS  WITH  FRESH  AND  DECOMPOSED 
SLUDGE  AT  THE  ESSEN  EXPERIMENTAL  PLANT. 

The  results  of  the  analyses  were: 


Water 

Dried 
material 

Ash 

Loss  by 

ignition 

92/48% 
Of  the  dried  material 

7.52% 

3.21% 

42  7  % 

4.31%\ 
57.3  %  f 

Fresh 
sludge. 

89.44% 
Of  the  dried  material 

10.56% 

6.13% 
58  .  05% 

4.43%  1 

41.95%  / 

Decomposed 
sludge. 

Forty-four  pounds  (20  kg.)  of  each  kind  of  sludge  was  brought 
to  the  draining  beds.  It  was  found  that  no  comparison  could  be 
made  in  this  way,  as  the  fresh  sludge  drained  off  immediately 
through  the  beds,  while  the  decomposed  sludge,  as  usual,  only 
permitted  clear  drainage  water  to  pass.  As  the  two  kinds  of 
sludge  did  not  differ  greatly  as  to  the  water  contained,  this  was 
rather  surprising.  The  decomposed  sludge  looked  somewhat 
thicker  than  the  other.  As  was  seen  later  this  was  caused 
less  by  concentration  than  by  the  gas  contained.  Decomposed 
sludge  which  is  full  of  gas  bubbles  becomes  foamy  and  viscous. 

It  was  thus  impossible  to  compare  the  two  sludges  as  to  ease  of 
draining. 

Fresh  sludge  may  be  concentrated  to  a  considerable  degree, 
as  a  large  part  of  the  water  rises  in  a  fairly  clear  condition  to  the 
surface  after  standing  awhile  and  can  then  be  drawn  off  by  a 
siphon.  Such  a  process  is  advantageous  in  two  respects.  In  the 
first  place,  it  is  probably  possible  in  this  way  to  drain  the  sludge 
without  its  running  through,  and  secondly,  it  was  possible  to 
assume  approximately  equal  amounts  of  moisture,  which  is  of 
importance  in  considering  the  quantity  of  the  run-off  measured 
in  the  process  of  draining.  A  direct  comparison  of  the  time 
taken  for  drying  had  to  be  abandoned  under  the  conditions,  as 
the  fresh  sludge  was  favored  by  the  removal  of  a  part  of  its 
moisture. 


154  SEWAGE  SLUDGE 

COMPARATIVE    EXPERIMENTS   WITH   FRESH   AND    DECOMPOSED 
SLUDGE  AT  THE  RECHLINGHAUSEN-OST  PLANT 

In  order  to  place  the  work  on  a  broader  basis,  sludge  from 
another  plant — that  at  Rechlinghausen-Ost1 — was  used,  which 
was  constructed  on  the  principle  of  the  Emscher  tanks.  This 
furnishes  a  more  concentrated  decomposed  sludge  in  its  well  23 
ft.  (7  m.)  in  depth  than  that  from  the  experimental  plant,  11  1/2 
ft.  (3.5  m.)  deep,  which  was  not  originally  built  as  an  Emscher 
tank. 

In  order  to  experiment  with  samples  containing,  so  far  as 
possible,  an  equal  amount  of  moisture,  fresh  sludge  was  allowed 
to  settle;  the  supernatant  water  was  siphoned  off  and  the  com- 
parative experiments  were  begun.  Analyses  of  the  two  kinds  of 
sludge  now  showed  that  the  desired  equality  of  the  contained 
waters  had  not  been  secured.  The  decomposed  sludge  contained 
77.4  per  cent,  of  moisture,  about  the  average  at  that  time  (77.6 
per  cent.).  The  fresh  sludge  was  not  equally  concentrated. 
The  moisture  (about  90  per  cent.)  had  been  reduced  by  siphoning 
but  it  still  amounted  to  80.35  per  cent.  Further  examination 
showed  that  in  other  respects  the  compositions  of  the  two  kinds 
of  sludge  were  not  comparable,  as  the  ash  in  the  dried  material 
of  the  fresh  sludge  was  57.2  per  cent.,  while  the  decomposed 
sludge  contained  but  50.58  per  cent. 

Both  differences  were  favorable  to  the  fresh  sludge,  for  wet 
sludge  will  give  off  more  moisture,  and  sludge  with  less  organic 
matter  dries  more  easily. 

In  the  experiment  described  the  attempt  to  measure  directly 
the  loss  in  weight  of  sludge  by  draining  had  to  be  abandoned. 
As  no  attempt  had  been  made  to  take  intermediate  samples,  this 
was  now  done. 

EXECUTION  OF  COMPARATIVE  EXPERIMENTS 

Forty-four  pounds  (20  kg.)  of  each  of  the  two  kinds  of  sludge 
were  again  weighed  and  placed  on  the  two  draining  beds. 

The  effluents  from  the  draining  beds  were  measured  daily  and 
analyzed  from  time  to  time.  The  beds  themselves  were  also 
weighed  at  intervals — 10  times  in  all.  The  results  of  measuring 

1  At  time  of  operation  in  Feb.,  1907,  about  28,000  inhabitants  provided  for.  5400  cbm. 
per  day.  Six  tanks  6  m.  in  diameter  and  7  m.  deep. 


DRYING  OF  SLUDGE 


155 


and  weighing  are  shown  graphically  in  Fig.  32.     This  shows  the 
summation  of  the  volumes  of  effluent  and  the  loss  of  weight. 

It  shows  how  the  fresh  sludge  which  drained  more  quickly  in 
the  beginning  on  account  of  the  large  amount  of  moisture  con- 
tained, was  surpassed  by  the  decomposed  sludge  on  the  second 
day.  The  difference  increased  up  to  the  4th  day  (April  29,  1908). 
Up  to  that  time  a  thin  layer  of  water  rested  on  the  surface  of  the 
fresh  sludge.  As  the  sludge  underneath  decreased  in  volume 
as  the  water  was  given  off,  contraction  cracks  appeared  through 
which,  in  the  two  following  days,  the  surface  water  could  seep. 
The  difference  in  the  amounts  of  effluent  was  thus  somewhat 


Volumes   of  Effluent  and   Diminution    in   Weight. 


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FIG.  32. — Drainage  of  fresh  and  decomposed  sludge  from  the  Recklinghausen-Ost 
clarification  plant. 

lessened.  Although  it  decreased  still  more  during  the  experi- 
ment, yet  at  the  end  it  amounted  to  0.24  gallons  (0.911 1.).  Only 
40  per  cent,  of  the  original  amount  of  fresh  sludge  appeared  as 
drainage  water  to  be  measured,  while  of  the  decomposed  sludge 
47.4  per  cent,  appeared. 

Results  of  Draining. — In  spite  of  the  greater  amount  of  mois- 
ture, the  fresh  sludge  gave  up  less  water  than  decomposed  sludge, 
even  on  this  freshly  prepared  drying  bed,  i.e.,  its  drainage  was 
inferior  to  that  of  the  decomposed  sludge.  (As  later  experiments 
showed,  the  difference  in  practice  is  still  greater,  because  the 
drying  beds  become  clogged  by  the  fresh  sludge  placed  upon  it. 
Decomposed  sludge  does  not  clog  the  beds.) 

The  curve  showing  the  reduction  in  weight  is  similar  to,  but 
differs  from  the  volume  of  the  effluent  in  that  it  includes  the 
reduction  by  evaporation  and  by  gasification.  Moreover,  as  the 


156  SEWAGE  SLUDGE 

latter  is  greater  in  the  case  of  decomposed  sludge,  the  difference 
is  greater  than  it  would  otherwise  be. 

Examination  of  Drainage  Water. — Results  of  the  examination 
of  the  drainage  water  are  given  in  Table  VII.  The  effluents 
of  the  first,  second  and  sixth  days  of  fresh,  as  well  as  decom- 
posed, sludge  were  examined  and  compared.  Effluents  of  both 
kinds  were  found  to  be  biologically  pure — i.e.,  they  contained 
and  produced  no  sulphuretted  hydrogen,  but  did  contain  nitrates 
and  nitrites. 

The  difference  between  the  two  was  best  shown  in  the  residue 
on  evaporation,  and  by  its  percentage  of  organic  matter,  which 
was  much  greater  in  fresh  than  in  decomposed  sludge.  The 
oxidability,  according  to  Kubel,  shows  the  same  thing. 

Decomposed  sludge  therefore  gives  off  drainage  water  of  a 
more  favorable  composition  than  fresh  sludge. 

Drying. — Placing  the  beds  under  cover  of  a  roof  furnished  an 
ideal  condition  for  draining  and  for  comparison  but  not  for 
rapidity  of  drying.  Although  the  deterring  influence  of  rain 
was  removed,  evaporation  was  hindered  by  the  lack  of  air  and 
sunshine.  The  desired  data  for  a  comparison  of  the  facility  of 
draining  was  obtained  but  the  time  required  for  drying  was 
greatly  increased.  Decomposed  sludge  became  spadable  in 
16  days,  which  is  a  short  time.  Fresh  sludge  reached  a  similar 
consistency  only  after  33  days,  although  it  had  been  artificially 
dried — more  than  twice  as  long  as  the  other.  After  the  fresh 
sludge  had  become  firm,  the  experiment  was  concluded  and  the 
average  samples  were  analyzed.  Table  VIII  gives  the  results 
of  the  analyses  from  samples  taken  before  beginning  and  after 
ending  the  experiment. 

RESULTS 

A.  Fresh  Sludge. — We  may  conclude  from  the  experiments 
that,  for  fresh  sludge: 

1.  If    it    is    brought    without    preliminary    concentration    to 
freshly  prepared  drying  beds,  it  is  possible  that  it  may  run  through 
without  being  drained,  even  with  a  surface  composed  of  grains 
2  to  4  mm.  in  size. 

2.  If,  as  in  the  second  experiment,  the  sludge  has  been  con- 
centrated (it  contained  only  80.35  per  cent,  moisture)  it  can  be 
drained  on  freshly  prepared  beds.     (Practically  it  has  at  this 


DRYING  OF  SLUDGE 


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158 


SEWAGE  SLUDGE 


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DRYING  OF  SLUDGE  159 

time  no  value;  first,  because  it  is  not  practicable  to  obtain  large 
amounts  of  fresh  sludge  with  much  less  than  90  per  cent,  mois- 
ture; secondly,  because  the  draining  beds  become  clogged  after 
fresh  sludge  has  been  placed  6n  them).  As  a  rule  also,  it  gives 
out  an  unbearable  odor,  noticeable  at  a  long  distance,  after 
about  3  days. 

3.  Drainage  water  from  fresh  sludge  becomes  biologically 
pure  after  slowly  percolating  through  the  layer  of  slag,  but 
still  contains  much  organic  matter  in  solution. 

B.  Decomposed  Sludge. — The  loss  of  water  by  draining  de- 
composed sludge  was  80.9  per  cent,  of  the  total  loss  of  moisture, 
so  that  only  19.1  per  cent,  of  that  lost  was  evaporated. 

C.  Comparison   of  Fresh  and  Decomposed  Sludge. — A   com- 
parison by  draining  the  two  kinds  of  sludge  shows  that: 

1.  Fresh  sludge  takes  much  longer  to  become  spadable  than 
decomposed  sludge,  even  when  it  is  at  first  partly  de-watered 
(33  as  compared  with  16). 

2.  Fresh  sludge  gives  off  much  less  drainage  water  than  de- 
composed sludge,  even  when  it  contains  more  moisture  (47.45 
per  cent,  as  compared  with  55.7  per  cent.). 

3.  "Drainage    water   from    decomposed    sludge    contains   less 
organic  matter  than  from  fresh  sludge. 

4.  Decomposed  sludge  loses  more  organic  matter  by  drainage 
than  fresh  sludge. 

DEDUCTIONS 

Provision  should  be  made  for  drainage  in  constructing  drying 
beds  for  decomposed  sludge.  The  drain  pipes  should  be  large 
enough  to  furnish  an  unobstructed  flow  for  the  large  amounts 
of  effluent  at  the  beginning. 

Drainage  water  from  beds  of  fine  slag  requires  no  further 
treatment.  It  can  be  discharged  into  any  stream. 

THE  REASONS  FOR  FACILITY  IN  DRAINING 

The  principal  result  of  the  experiments  described  was  establish- 
ing the  fact  that  decomposed  sludge  from  Emscher  tanks  can  be 
drained,  i.e.,  it  dries  in  a  short  time  by  parting  with  a  large  part 
of  the  water  which  disappears  (to  80  per  cent.)  through  the  porous 
bed.  It  remains  to  find  out  what  characteristics  are  required  to 
render  drainage  easy. 


160  SEWAGE  SLUDGE 

Viscosity. — The  experiments  with  fresh  and  decomposed  sludge 
furnished  very  important  information.  They  showed  that  with 
a  cover  to  the  bed  of  uniform  sized  grains  fresh  sludge  ran  through, 
while  decomposed  sludge  merely  lost  its  moisture.  The  reason 
lies  in  the  difference  in  concentration. 

With  fresh  sludge  only  the  coarsest  ingredients  remain  on  the 
drainage  surface  at  first,  owing  to  its  fluid  state,  due  to  the  large 
amount  of  contained  water,  while  the  finer  material  in  part 
penetrates  to  a  greater  or  less  depth  into  the  covering  layer, 
some  of  it  passing  all  the  way  through.  The  layer  becomes  more 
dense  by  the  accumulation  of  the  particles  of  sludge  which  pene- 
trate the  surface,  no  more  passes  through  and  at  a  certain  depth 
an  almost  impenetrable  mass  is  formed  of  the  covering  material 
and  the  particles  of  sludge  which,  when  the  second  or  third  dose 
of  sludge  is  applied  may,  under  some  circumstances,  become  so 
thick  that  it  offers  a  strong  resistance  to  the  passage  of  water. 
There  can  then  be  no  question  of  draining  through  so  impervious 
a  bed. 

The  more  concentrated  septic  sludge^  and  the  thick-flowing 
digested  sludge  of  Emscher  tanks,  on  the  contrary,  do  not  pass 
through  the  filtering  layers,  but  give  off  their  water. 

Destruction  of  Colloids. — According  to  the  results  of  the  second 
experiment  Emscher  sludge  drained  more  rapidly  than  artificially 
concentrated  fresh  sludge  from  the  same  plant.  This  is  due 
largely  to  the  fact  that  the  colloids  in  the  fresh  sludge  are  partially 
destroyed  (in  digested  sludge — Trans.) .  Presumably  the  decom- 
position caused  by  bacteria  and  enzymes  which  attack  the  organic 
material  on  the  surface,  is  most  apparent  in  the  sponge-like 
hydrogels  filled  with  liquid  with  their  enormously  large  surfaces. 
The  destruction  of  these  diminishes  their  property  of  holding 
water.  As  the  sludge  loses  moisture  it  drains  more  easily. 

Destruction  of  Organic  Matter. — The  destruction  of  organic 
matter,  such  as  fragments  of  animals  and  plants,  which  are  found 
in  the  sewage  from  kitchen,  garden  and  slaughter  house  wastes, 
takes  place  in  the  same  way.  These  substances,  which  bind,  and 
from  the  beginning  contain,  much  water,  are  found  only  in  very 
small  quantities  in  decomposed  sludge. 

The  difference  in  the  water  content  between  fresh  and  decom- 
posed sludge  shows  how  far  the  destruction  of  the  water  binding 
colloids  and  organic  matter  has  progressed.  As  compared  with 
90  to  95  per  cent,  moisture  in  fresh  sludge,  I  found,  e.g.,  in 


DRYING  OF  SLUDGE  161 

Emscher  sludge  from  the  Recklinghausen  plant  an  average  of 
79.3  per  cent.,  in  sludge  from  the  Essen-N.  W.  plant  only  75.6 
per  cent.  Occasionally  liquid  sludge  is  obtained  from- Emscher 
tanks  with  nearly  as  little  as  70  per  cent,  moisture,  a  concentra- 
tion equal  to  the  spadable  sludge  from  centrifugal  machines. 
On  the  8th  of  April,  1909,  in  an  average  sample  of  123.58 
cu.  yds.  (94.34  cbm.)  of  wet  sludge  drawn  off  under  water,  from 
the  Essen-N.  W.  plant,  e.g.,  I  found  71.9  per  cent,  moisture;  from 
another  taken  May  8, 1909,  of  85.1  cu.  yds.  (65.0  cbm.),  71.35  per 
cent,  moisture,  while,  according  to  examinations  made  at  the 
Royal  Experimental  Station  for  Water  Supply  and  Sewage 
Disposal1,  the  spadable  centrifuged  sludge  from  Harburg  con- 
tained between  69.7  and  74.2  per  cent,  moisture,  an  average  of 
72.5  per  cent.  A  sample  taken  by  me  at  Harburg  in  1908 
showed  68.8  per  cent.  In  Frankfort  the  fresh  centrifuged  sludge 
contained  about  70  per  cent,  moisture  (according  to  data  fur- 
nished me  there  in  1908). 

The  destruction  of  water  binding  substances  is  shown  also 
when  in  a  spadable  condition. 

Thus  I  found  in  4  samples  of  Emscher  sludge  at  Reckling- 
hausen-Ost  which  had  just  become  spadable,  an  average  of 
58.27  per  cent.,  in  13  samples  from  Essen-N.  W.  an  average  of 
52.34  per  cent. 

We  may  thus  assume  for  spadable  fresh  sludge  about  71  per 
cent,  moisture,  for  spadable  decomposed  sludge  about  55  per 
cent. 

Gas  Contained. — The  ability  of  decomposed  sludge  to  drain  is 
materially  assisted  by  the  gas  contained.  As  already  mentioned, 
large  quantities  of  gas  are  formed  by  the  decomposition  of  sludge 
in  Emscher  tanks,  which  consists  mainly  (about  3/4)  of  methane 
and  (about  1/4)  of  carbonic  acid.  These  gases  pass  off  as  soon  as 
large  bubbles  are  formed  from  the  original  minute  ones,  as  the 
former  overcome  the  pressure  of  the  overlying  layer  of  sludge. 
A  large  volume  of  gas  at  one  point  is  necessary  to  effect  this. 
The  bubbles  remain  in  the  viscous  material  until  this  occurs. 
At  the  greatest  depth,  about  33  ft.  (10  m.)  the  gases  are  under  a 
pressure  of  one  atmosphere.  If  sludge  is  drawn  off  it  comes 
filled  with  compressed  gas.  With  the  release  of  pressure  from 
this  greater  depth  the  volume  of  the  bubbles  increases,  increasing 

1  Reichle  and  Thiesing  "Mitteilung  aua  der  Kgl.  Pruf-Anst.  fur  Wasserv.  und  Abwasser- 
beseitigung,"  No.  10. 
11 


162 


SEWAGE  SLUDGE 


the  volume  of  the  sludge.  This,  being  full  of  these  gas  bubbles, 
is  changed  into  a  foaming  mass.  The  increase  in  volume  renders 
drainage  more  easy.  The  tendency  to  penetrate  the  surface 
layer  of  the  beds  is  reduced.  The  water  in  the  sludge  passes 
through  the  channels  formed  by  the  disappearing  gas,  seeps 
down  and  drains  off.  If  sludge  freshly  drawn  from  an  Emscher 
tank  is  allowed  to  stand  in  a  glass  cylinder  (which  may  be  con- 
sidered an  impervious  sludge  bed)  bubbles  of  gas  may  be  seen 
on  the  sides  which  gradually  increase  in  number  and  size.  The 
volume  then  increases  and  a  layer  of  clear  water  forms  at  the 
bottom.  The  volume  of  sludge  above  the  water  is  not  reduced  as 


FIG.  33. 


FIG.  34. 


FIG.  35. 


it  has  a  foamy  structure,  i.e.,  it  contains  a  great  number  of  small 
and  large  compartments  filled  with  gas  bubbles.  This  foamy 
material,  being  lighter  than  water,  is  forced  up  by  the  water 
which  settles  to  the  bottom.  It  also  spreads  as  the  gas  increases 
in  volume.  The  volume  of  the  whole  is  thus  increased  by  more 
than  that  of  the  water  at  the  bottom. 

Figs.  33  and  34  illustrate  the  process.  The  original  height  of 
the  sludge  is  indicated  by  the  upper  edge  of  the  strip  of  paper. 
The  second  cylinder  shows  how  the  water  has  settled  after  24 
hours  and  the  entire  volume  is  increased.  The  thin  layer  of 
sludge  at  the  bottom  is  composed  of  heavy  particles  which  are 
deposited  later,  as  shown  by  the  solid  particles  just  sinking. 


DRYING  OF  SLUDGE 


163 


This  method  increases  the  ease  of  drainage  appreciably.  The 
water  can  filter  through  unhindered  as  it  reaches  the  porous  cover- 
ing containing  no  sludge. 

Fresh  sludge  is  just  the  reverse  in  this  respect.  The  water 
does  not  sink,  but  rises.  Fig.  35  represents  fresh  sludge  from  the 
clarification  plant  at  Essen. 24  hours  after  being  placed  in  the 
cylinder.  The  upper  edge  of  the  paper  indicates  again  the  orig- 
inal surface  of  the  sludge.  It  can  be  seen  that  the  sludge  has 
not  risen  and  that  dirty  water  is  on  the  top. 

In  order  to  show  to  what  extent  the  enclosed  gases  cause 
these  phenomena  in  decomposed  sludge  and  whether  a  subse- 
quent evolution  of  gas  assists,  I  experimented  with  a  sample  of 
sludge  from  Essen-N.  W.  by  warming  a  portion  of  it  for  two 
hours  at  99°  F.  (37°  C.) ,  and  stirring  it  frequently  to  remove  the 
larger  gas  bubbles  and  then  removing  the  gas  as  much  as  possible 
by  subjecting  it  to  the  vacuum  produced  by  a  good  ejector  while 
shaking  it  frequently.  The  sludge  sample  so  treated  and  an- 
other without  having  the  gas  removed  were  allowed  to  stand  24 
hours  with  as  uniform  a  temperature  as  possible. 

The  original  amount  and  the  increase  in  volume  and  the  set- 
tled water  were  measured. 


TABLE  IX 
SLUDGE  FROM  TANK  No.  5  OF  THE  EssEN-N.  W.  PLANT 


1.  In  original  condition 

2.  With  gas  removed 

Gals. 

c.c. 

Gals. 

c.c. 

Quantity    at    beginning    of 

0.1096 

4151 

0.0832 

315 

experiment. 

After  24  hours                           1          O  1  4OO 

530 

0  0937 

3552 

Increase  in  volume  

0.0304 

115 

0.0105 

40 

Same    in     per    cent,     of  1 
original  amount                  J 

27.7  below. 

12.7  above. 

Water  separated  

0.0105 

40 

0.0040 

15 

Same    in     per    cent,     of  ) 
original  amount.                 / 

9.64 

4.763 

The  temperature  at  the  beginning  was  60.8°  F.  (16.0°  C.)  and 
at  the  end  59.9°  F.  (15.5°  C.). 

1  425  in  original.     2  455  in  original.     3  4 . 3  in  original.     The  foregoing  alterations  made 
to  secure  consistent  results  as  the  original  figures  are  erroneous.     Tr. 


164  SEWAGE  SLUDGE 

It  is  an  astonishing  fact  that  in  spite  of  the  gas  removed  there 
was  a  perceptible  increase  in  volume,  although  this  was  not 
quite  half  so  large  as  before.  It  is  possible,  either  that  the  gas 
was  not  entirely  removed  and  that  a  thicker  more  watery  layer 
had  accumulated  in  the  lower  part  of  the  sludge,  forcing  up  the 
lighter  parts,  or  else  that,  on  account  of  the  action  of  bacteria 
and  enzymes  on  the  sludge,  there  was  a  further  development  of 
gas,  thus  increasing  the  volume.  Probably  both  these  views 
are  true. 

EXPERIENCE  IN  METHODS  OF  DRAINING  AT  LARGE  PLANTS 

These  experiments  show  that  with  proper  preliminary  treat- 
ment (decomposition  under  water  in  deep  tanks)  sludge  on 
drained  drying  beds  may  be  easily  separated  into  a  spadable 
earthy  material  and  a  harmless  liquid  which  has  the  character- 
istics of  an  effluent  from  contact  beds.  These  experiments, 
although  successful  on  a  small  scale,  do  not  solve  the  question  as 
to  the  applicability  of  this  method  to  a  large  plant. 

Drying  sludge  by  draining  has  been  practised  on  a  large  scale 
at  Recklinghausen-Ost  (28,000  inhabitants),  Essen-N.  W. 
(60,000  inhabitants)  and  Bochum  (130,000  inhabitants),  as  well 
as  at  several  smaller  plants.  The  results  have  been  much  more 
favorable  than  was  anticipated.  Much  material  is  available 
regarding  the  results  of  drying  at  the  more  accessible  of  the  two 
large  plants,  that  at  Essen-N.  W.,  collected  by  operating  engineer 
Blunk,  in  so  far  as  it  relates  to  the  measurement  of  the  depth  of 
sludge  and  the  time  of  drying,  in  connecteon  with  estimates  for 
the  contractors  for  the  removal  of  the  sludge  authorized  by  the 
Emscher  Association.  These  measurements  have  been  shown 
in  diagrams  (not  reproduced).  The  rainfall  was  measured  by 
Mr.  Winter,  Municipal  Superintendent  of  Clarification,  at  the 
Essen  plant. 

Description  of  Drying  Beds. — The  drying  bed  at  Essen-N.  W. 
for  sludge  taken  from  a  tank  29.5  ft.  (9  m.)  deep  lies  several 
meters  below  the  surface  of  the  ground  and  is  artificially  drained 
by  an  underground  conduit  laid  parallel  with  the  stream  to 
which  it  empties,  and  has  an  outlet  below  a  dam.  The  bed  is 
supplied  with  drain  pipes  laid  end  to  end,  at  intervals  of  8.2  to 
9.8  ft.  (2.5  to  3  m.).  These  lead  to  an  open  ditch  surrounding 
the  bed.  Above  the  pipes  is  a  layer  of  furnace  slag  12  in.  (30  cm.) 
thick,  and  above  this  a  layer  of  crushed  slag  8-10  in.  (2  cm.) 


DRYING  OF  SLUDGE 


165 


thick.  (Coke  cinders  are  sometimes  used  in  place  of  the  latter, 
as  they  are  often  cheaper).  In  recent  plants  having  a  sand 
catcher  the  grit  taken  from  this  is  used.  As  this  contains  no 
floating  particles  it  need  only  be  placed  in  thin  layers. 

The  drying  bed  is  divided  longitudinally  by  planks  into  3  parts, 
numbered  I,  II  and  III.  They  are  3465,  3411  and  3153  sq.  ft. 
(322,  317,  and  293  sq.  m.)  in  size.  Each  has,  along  the  middle, 
rails  supported  by  piles  for  carrying  the  sludge  away.  The 
plant  began  operation  in  December,  1908.  Sludge  was  placed 
on  beds  I  and  III  April  8,  1909,  and  on  bed  II  April  10,  1909. 
In  a  short  time  (3  to  5  days)  it  became  spadable,  but  was  not 
removed  from  the  beds  until  April  19,  1909.  (Time  of  retention 
9  to  11  days.) 

NOTE  BY  TRANSLATOR 

In  the  conclusions  drawn  from  these  experiments  no  con- 
sideration has  been  given  to  these  first  drainings,  as  it  could  not 
be  determined  definitely  when  the  sludge  became  spadable.  The 
results  that  were  reached  cover  the  period  of  a  full  year,  ending 
May  1,  1910. 

The  septic  sludge  received  by  the  drying  beds  was  as  follows: 


Month,  1909 

Bed  number 

I 

II 

III 

Total 

Cu.  yds. 

Cbm. 

Cu.  yds. 

Cbm. 

Cu.  yds. 

Cbm. 

Cu.  yds. 

Cbm. 

May  
June  
July  
August  

355.3 
358.1 
271.7 
278.1 

271.6 
273.8 
207.7 
212.6 

424.1 
264.8 
201.2 
337.2 

324.2 
202.4 
153.8 
257.8 

360.4 
337.3 
175.5 
326.5 

275.5 
257.9 
134.2 
249.6 

1139.8 
960.2 
648.4 
941.8 

871.3 
734.1 
495.7 
720.0 

The  total  for  June,  July  and  August  was  about  2550  cu.  yds. 
(1950  cbm.)  in  92  days,  giving  an  average  of  27.7  cu.  yds. 
(21.2  cbm.)  of  septic  sludge  per  day  containing  about  80  per  cent, 
moisture. 

According  to  Spillner  and  Blunk1  the  mean  daily  flow  of 
sewage  to  the  Essen-N.  W.  plant  was,  at  the  time  under  con- 
sideration (1909-10),  as  follows: 

1  Tech.  Semeind.,  Vol.  XIII  (1910). 


166  SEWAGE  SLUDGE 

Sewage  from  60,000  persons  ...   2.77J  mil.  gal.  =  10,500  cbm.      =32.0% 

Wastes  from  Krupp's  works  ...    9.51  mil.  gal.  =36,000  cbm.      =63.7% 

Mine  drainage 0.26|  mil.  gal.=     1000  cbm.  \  _ 

Coal  washing  water 0. 13  mil.  gal.  =       500  cbm.  /  = 

Total1 12.68~  mil.  gal.  =48,000  cbm.   =  100. 0% 

If  we  assume  this  volume  equivalent  to  that  derived  from  a 
population  of  65,000,  we  have: 

Volume  of  sewage  per  capita  daily 194.8      gallons  =738          lit. 

Volume  of  wet  sludge  per  thousand  persons 

daily 426  cu.  yds.  =       .326  cbm. 

Volume  of  wet  sludge  per  million  gallons 

sewage 2.19    cu.  yds. 

Volume  of  wet  sludge  per  cubic  meter  sewage,  =        .  442  lit. 

The  results  of  operation  show  that  the  expectations  from  the 
methods  adopted  for  drainage  have  been  realized.  In  spite  of 
an  unusually  wet  year,  sludge  averaging  9  in.  (23  cm.)  in  depth, 
became  spadable  in  5.87  days.  Sometimes,  in^dry  weather,  it 
dried  in  2  days,  as  May  11  (III)  and  May  20  (II);  in  September 
sometimes  in  one  day. 

In  the  365  days  under  consideration,  the  drying  beds  were: 

I     Occupied  236  days         Empty  129  days. 

II     Occupied  246  days         Empty  119  days. 

Ill     Occupied  294  days         Empty    81  days. 

These  figures  show  that  the  beds  were  not  completely  utilized 
although  their  area  was  but  about  9684  sq.  ft.  (900  sq.  m.), 
making,  for  a  population  of  60,000,  only  0.161  sq.  ft.  (0.015  sq. 
m.)  per  capita.  The  tables  show,  moreover,  that  sludge  is 
seldom  removed  in  winter.  As  it  takes  longer  to  decompose  and 
to  dry  in  winter  than  in  summer,  care  was  taken  to  provide  as 
much  storage  capacity  as  possible  in  the  deep  sludge  chambers  in 
winter.  In  this  way  one  is  independent  of  the  weather,  as  it  is 
only  necessary  to  discharge  small  quantities  of  sludge,  and  these 
at  long  intervals. 

Table  X  shows  the  changes  which  sludge  undergoes  in  drain- 
ing. The  amount  was  reduced  45  to  58  per  cent,  in  weight,  60 
to  77  per  cent,  in  moisture  and  0.1  to  0.9  per  cent,  in  dried 
material. 

1  The  Krupp  wastes  contain  the  sewage  and  water  used  in  lavatories  from  about  10,000 
workmen.  The  sewers  are  on  the  combined  system.  The  disposal  works  consist  of  two 
grit  chambers,  one  coarse  screen  and  nine  Emscher  tanks.  These  were  built  in  1907-8 
and  put  in  operation  November,  1908. 


DRYING  OF  SLUDGE 


167 


• 

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SEWAGE  SLUDGE 


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DRYING  OF  SLUDGE 


169 


Thirteen  analyses  gave  an  average  of  52.3  per  cent,  moisture 
in  spadable  sludge. 

Drainage  Water. — Drainage  water,  of  which  several  samples 
were  taken,  showed  the  same  favorable  characteristics  as  in  the 
experiments  described. 

24-Hour  Samples. — In  order  to  avoid  any  accidental  errors  in 
sampling  and  in  order  to  show  the  changing  composition  of  the 
drainage  water  in  the  course  of  the  experiment,  continuous 
average  samples  were  taken  on  August  12  and  13,  1909.  The 
samples,  which  were  taken  hourly  from  all  of  the  15  effluent 
drains,  were  mixed  together  for  each  3-hour  sample  and  ex- 
amined. The  result  of  these  analyses  is  given  in  Table  XI. 

TABLE  XI 

ESSEN-NORDWEST    CLARIFICATION    PLANT 

Drainage  Water,  August  12  and  13,  1909,  from  Bed  III 


Time 

10.15 
to 
1.15 

1.15 
to 
4.15 

4.15 
to 
7.15 

7.15 
to 
10.15 

10.15 
to 
1.15 

1.15 
to 
4.15 

4.15 
to 
7.15 

7.15 
to 
10.15 

Gallons  per  period1  .  . 
Liters  per  period1  .  .  . 

Residue  on  evapora- 
tion   
Residue  on  ignition  .  . 
Loss  on  ignition  
Chlorine 

296.0 
1,120.4 

2,713.5 
2,298.5 
415.0 
426.0 
47.6 
21.0 
0.7 

25.9 

79.6 
53.0 
26.6 

Not 
putresc. 

193.2 
731.4 

2,831.5 
2,440.5 
391.0 
488.0 
47.6 
23.8 
1.4 

22.4 

91.0 
64.8 
26.2 

Not 
putresc. 

186.4 
705.5 
Parts  p 

2,935.5 
2,491  .  5 
444.0 
520.0 
53.2 
23.1 
3.5 

26.6 

69.6 
50.0 
19.6 

Not 
putresc. 

164.9 
624.0 
er  Millio 

3,012.0 
2,578.5 
433.5 
544.0 
47.6 
24.5 
1.4 

21.7 

44.4 

27.8 
16.6 

Not 
putresc. 

142.7 
540.7 
n=Mg 

2,863.0 
2,557.5 
305.5 
556.0 
49.7 
30.1 
2.8 

16.8 

46.2 
31.8 
14.4 

Not 
putresc. 

123.0 
465.6 
per  liter 

2,713.0 
2,427.5 
285.5 
560.0 
51.8 
35.0 
2.1 

14.7 

57.8 
41.6 
16.2 

Not 
putresc. 

88.9 
336.6 

2,612.5 
2,286.5 
326.0 
556.0 
51.8 
36.4 
0.7 

14.7 

194.2 
145.4 
48.8 

Not 
putresc. 

83.7 
316.7 

2,560,0 
2,229.0 
331.0 
540.0 
49.0 
33.6 
1.4 

14.0 

159.6 
116.8 
42.8 

Not 
putresc. 

Nitrogen,  total  
Nitrogen,  as  H4N.  .  .  . 
Organic  nitrogen  .... 
N    in    nitrates    and 
nitrites  
Suspended      matter, 
total 

Mineral 

Organic  
Putrescibility  10  days 
storage   in   closed 
flask. 

It  shows  that  the  drainage  water  meets  the  demands  of  a 
biologically  pure  water,  for  the  nitrogen  is  almost  entirely 
mineralized  and  the  liquids  show  no  signs  of  putrefaction  (H2S 
reaction)  even  when  kept  for  10  days  in  a  closed  bottle. 

1  The  word  "Stunde"  is  taken  to  mean  period  rather  than  hour. — Tr. 


170  SEWAGE  SLUDGE 

Drainage,  therefore,  fulfills  all  expectations  from  the  experi- 
ments as  to  time  of  drying  and  composition.  It  is  employed  at 
the  six  plants  now  in  operation  in  the  Emscher  District. 

Removal  of  Drained  Sludge. — At  one  of  these  plants  (Reck- 
linghausen-Ost)  the  drained  sludge  is  sold  as  a  fertilizer  to  the 
farmers  at  12  cts.  (50  pfg.)  per  cartload  (at  the  dumping  ground). 
In  the  3  years  during  which  this  plant  has  been  in  operation,  the 
demand  has  exceeded  the  supply,  so  that  the  sludge  is  usually 
sold  long  before  it  is  prepared. 

When  no  farming  is  carried  on  in  the  neighborhood,  drained 
sludge  is  used  for  filling  in  land.  It  is  particularly  well  adapted 
to  this  as  it  does  not  soften  with  the  rain  and  is  so  firm  that  large 
deposits  of  it  can  be  walked  on  without  sinking. 

At  Essen-Nord,  which  was  built  this  year,  attempts  will  be 
made  to  dry  the  drained  sludge  from  180,000  inhabitants  in 
furnaces,  similar  to  those  used  for  the  incineration  of  street 
sweepings. 

In  the  15  plants  to  be  built  this  year  in  the  Emscher  District, 
draining  beds  for  drying  the  sludge  have  been  planned  and  are, 
in  part,  constructed. 


RESULTS  OF  THE   OPERATION   OF 
SOME  OF  THE  MECHANICAL  SEW- 
AGE CLARIFICATION  PLANTS 
OF  THE  EMSCHER  ASSO- 
CIATION 

BY 

DR.  ING.  F.  SPILLNER 


CHEMIST 


AND 


MR.  BLUNK 

OPERATING    ENGINEER,    OP    THE    ASSOCIATION 

TRANSLATED   BY 

E.  KUICHLING,  C.  E. 


INTRODUCTORY  NOTE 

The  foregoing  paper  by  Dr.-Ing.  Spillner  presents  in  a  com 
pact  form  the  results  arrived  at  by  the  Emscher  Association  in 
the  operation  of  the  type  of  sedimentation  tank  devised  by  Dr. 
Ing.  Imhoff  for  the  Association  up  to  the  end  of  1909.  The 
viewpoint  taken  is,  naturally,  that  of  the  scientist,  and  the 
conclusions  drawn  are  largely  from  chemical  and  physical  tests 
of  samples  taken  during  the  operation  of  the  several  treatment 
plants. 

This  paper  is  now  very  appropriately  supplemented  by  one 
bringing  the  subject  up  to  the  present  year  written  by  Dr.-Ing. 
Spillner  in  conjunction  with  Mr.  Blunk,  the  engineer  in  charge 
of  operation.  This,  therefore,  not  only  has  the  advantage  of 
•a  longer  experience  with  this  mode  of  treating  sewage,  and  of 
the  various  comments  and  criticisms  concerning  the  Emscher 
tank  that  have  been  made  during  the  past  year  or  two  and 
which  are,  in  effect,  answered  in  this  way,  but  of  the  additional 
opinions  formed  by  one  intimately  connected  with  the  plants 
in  their  varying  conditions  of  actual  operation. 

More  or  less  matter  in  the  original  text  is,  almost  necessarily, 
a  repetition  in  new  form  of  what  was  contained  in  Spillner's 
original  paper.  So  far  as  consistent  with  the  proper  form  and 
interpretation  of  this  later  work  such  matter  has  been  omitted 
in  the  translation,  the  reader  being  referred  to  the  earlier  paper. 

K.  A. 


172 


RESULTS  OF  THE   OPERATION  OF  SOME  OF 
THE    MECHANICAL    SEWAGE   CLARIFI- 
CATION PLANTS  OF  THE  EMSCHER 
ASSOCIATION. 

MEASUREMENTS  OF  THE  SLUDGE 

The  quantity  of  sludge  contained  in  each  tank  of  the  sewage 
clarification  plants  at  Essen,  Recklinghausen  and  Bochum  was 
measured  at  intervals  of  ten  days  in  order  to  control  properly  the 
treatment  of  the  sludge.  These  measurements  were  made  by 
sounding  with  a  thin  sheet  iron  plate,  as  the  surface  of  the  sludge 
is  always  horizontal  and  compact  or  dense  enough  to  sustain  the 
weight  of  the  light  plate.  In  this  way  the  depth  of  the  super- 
incumbent liquid  was  readily  determined,  and  thence  also  the 
volume  occupied  by  the  sludge.  Any  decrease  in  this  depth 
represents  a  corresponding  increase  in  the  volume  of  sludge. 
The  quantity  of  water  contained  in  the  sludge  varies  at  different 
depths  in  the  mass,  but  the  error  in  measurement  caused  thereby 
is  balanced  in  the  successive  observations,  as  the  proportion  is 
approximately  constant.  These  soundings  are  entered  in  a 
special  book,  and  the  corresponding  volumes  of  sludge  are 
subsequently  computed  and  recorded  in  the  office. 

For  convenience  of  inspection  the  records  are  also  kept  in 
diagram  form,  with  the  ordinates  representing  the  total  quantity 
of  sludge  deposited  and  the  abscissas  the  number  of  days  elapsed 
from  the  outset.  The  discharge  of  sludge  from  each  tank  is 
similarly  recorded  and  shown  on  the  diagram.  By  properly 
connecting  the  successive  points  thus  located,  the  diagram  for 
each  plant  will  exhibit  two  more  or  less  irregular  lines,  of  which 
the  upper  one  shows  by  scale  the  total  volume  of  sludge  de- 
posited, while  the  lower  one  shows  the  total  volume  of  sludge 
discharged,  since  the  day  that  the  plant  was  put  in  operation; 
and  the  volume  of  sludge  contained  in  the  tanks  at  any  time 
will  be  represented  by  the  difference  between  the  ordinates  of 
the  upper  and  lower  line  for  that  time  or  day.  The  diagram 

173 


174 


SEWAGE  SLUDGE 


also  enables  us  to  determine  approximately  how  long  the  sludge 
which  was  removed  remained  in  the  tank.  This  is  done  by 
simply  moving  to  the  left  the  ordinate  of  the  lower  line  until 
it  coincides  with  the  equal  ordinate  of  the  upper  line,  and  noting 
the  corresponding  interval  on  the  axis  of  abscissas  which  shows 
the  number  of  days.  Several  such  diagrams  are  given  in  the 
paper,  but  with  one  exception  are  here  omitted. 

For  example,  the  diagram  for  the  Recklinghausen  plant 
shows  that  on  April  20,  1909,  the  total  deposit  of  sludge  was 
2609  cu.  yds.  (1995  cbm.),  and  that  on  the  same  day  59  cu.  yds. 
(45  cbm.)  were  discharged,  the  total  previous  discharge  having 


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1909  .......  - 


FIG.  36.  —  Diagram  showing  increase  of  sludge  in  the  Recklinghausen-Ost 

Clarification  Plant. 

The  upper  line  represents  the  aggregate  volume  of  sludge  deposited  and  the  lower  line 
represents  the  aggregate  volume  of  sludge  discharged. 


been  1910  cu.  yds.  (1460  cbm.) ;  hence  on  this  day  the  aggregate 
discharge  was  1910  +  59  =  1969  cu.  yds.  (1505  cbm.).  By 
moving  this  ordinate  to  the  left  until  it  intersects  the  upper 
line  representing  the  total  sludge  deposited,  it  will  be  found  that 
this  intersection  corresponds  on  the  axis  of  abscissas  to  Oct.  17, 
1908,  thus  indicating  that  the  sludge  which  was  removed 
on  April  20,  1909,  had  remained  in  the  tank  for  a  period  of  six 
months.  In  reality,  however,  the  period  of  detention  in  the 
tank  is  considerably  less,  as  the  sludge  does  not  descend  or  move 
at  a  uniform  rate  from  the  surface  to  the  mouth  of  the  discharge 
pipe  in  the  sump  at  the  bottom  of  the  tank. 


SEWAGE  CLARIFICATION  PLANTS  175 

Before  and  after  any  sludge  is  discharged  from  a  tank,  the 
position  of  its  surface  is  always  carefully  noted  by  soundings, 
as  above  described.  The  difference  between  these  two  measure- 
ments gives  the  quantity  removed,  which  is  checked  by  measur- 
ing the  depth  of  the  mass  at  a  number  of  places  upon  the  level 
drying  bed  or  filter.  If  made  quickly,  or  before  an  appreciable 
quantity  of  water  escapes  into  the  underdrains  of  the  bed,  the 
two  measurements  agree  closely. 

The  amount  of  water  in  the  discharged  sludge  varies  with  the 
depth  of  the  tank  and  the  age  and  chemical  composition  of  the 
sludge.  As  will  be  shown  subsequently,  it  contains  on  the 
average  about  75  per  cent,  of  water  as  it  leaves  the  tank;  and 
after  being  allowed  to  drain  for  a  few  days  upon  the  drying  beds, 
the  quantity  of  moisture  reduces  to  52  per  cent,  at  Essen  N.  W. 
and  65  per  cent,  at  Recklinghausen,  or  to  58  per  cent,  on  the 
average.  By  this  drainage  the  volume  of  the  sludge  is  reduced 
about  40  per  cent.,  and  it  then  becomes  consistent  enough  to  be 
spadable,  or  to  be  cut  and  handled  with  a  shovel 

The  authors  exhibit  in  diagram  form  the  results  attained  with 
the  sludge  of  the  Essen-N.  W.  plant  for  the  year  from  April  1, 
1909,  to  April  1,  1910.  There  are  three  separate  sludge  draining 
beds,  and  the  observations  relating  to  them  during  this  period 
are  shown  graphically.  The  several  lines  indicate  the  date  and 
quantity  of  sludge  discharged,  and  the  subsequent  date  and 
volume  when  the  sludge  had  become  spadable  and  was  removed 
from  the  drying  bed;  also  the  depth  of  the  rainfall  and  the  date 
of  its  occurrence.  When  first  taken  from  the  tanks  the  sludge 
contained  from  72  to  75  per  cent,  water,  and  when  finally  carried 
away  from  the  beds  it  contained  from  55  to  60  per  cent,  water. 
The  abscissas  indicate  the  number  of  days  required  for  the  sludge 
to  become  dry  enough  to  handle  with  a  shovel.  Thus  on  October 
6,  1909,  95.6  cu.  yds.  (73  cbm.)  of  liquid  sludge  was  discharged 
upon  bed  No.  I,  and  only  three  days  later  it  was  found  to  be 
spadable,  its  volume  having  reduced  to  53.1  cu.  yds.  (40.5  cbm.). 
In  this  short  period  the  shrinkage  in  volume  was  42.5  cu.  yds. 
(32.5  cbm.)  or  44.5  per  cent. 

Another  diagram  shows  the  accumulated  volumes  of  liquid  and 
drained  sludge  during  the  twelve  months  mentioned,  each  by  a 
continuous  line  or  curve.  It  shows  that  during  this  time  7194 
cu.  yds.  (5500  cbm.)  of  liquid  sludge  had  been  discharged  from 
the  tanks,  and  that  this  volume  had  been  reduced  by  drainage 


176  SEWAGE  SLUDGE 

to  4709  cu.  yds.  (3600  cbm.),  thus  making  the  average  shrinkage 
in  volume  35  per  cent.  The  preceeding  diagrams  also  indicate  a 
large  variation  in  the  time  required  for  the  sludge  to  become 
spadable,  but  the  reason  therefor  becomes  evident  on  comparing 
these  periods  with  the  corresponding  rainfall.  Thus  on  July  13 
and  14,  1909,  all  of  the  beds  had  been  filled  with  sludge,  and  in 
the  afternoon  of  the  fourteenth,  an  excessive  rainfall  occurred 
that  yielded  a  depth  of  1.34  in.,  and  by  the  failure  of  an  embank- 
ment caused  the  sludge  beds  to  become  covered  with  water  to  a 
depth  of  8  in.  In  consequence  of  this  accident  the  sludge  in  one 
of  the  beds  did  not  become  spadable  until  July  27,  a  period  of 
14  days,  although  a  much  shorter  time  sufficed  in  the  other  beds. 
Such  cases,  however,  are  exceptional,  and  the  average  period, 
including  rainy  weather,  is  from  6  to  7  days. 

In  dry  summer  weather,  the  drainage  or  drying  is  frequently 
accomplished  in  two  or  three  days,  while  in  severe  winter  weather 
a  somewhat  longer  time  is  required,  as  the  water  in  the  sludge 
may  then  freeze.  This  freezing  is  troublesome,  as  the  sludge  after 
thawing  is  not  only  rendered  nearly  as  wet  as  it  was  originally, 
but  is  also  deprived  of  its  contents  of  gases  upon  which  the 
facility  for  quick  drainage  depends  in  high  degree.  This  peculiar 
property  was  fully  pointed  out  in  a  paper  by  Dr.  Imhoff,  in 
Technisches  Gemeindeblatt,  October  5,  1910,  pp.  193-199.  In 
consequence  of  the  escape  of  the  gases  while  the  frozen  mass  is 
thawing,  the  wet  sludge  settles  upon  the  surface  of  the  bed,  theje- 
by  causing  it  to  become  clogged  and  compelling  the  water  to  rise 
to  the  top  of  the  liquid  mass,  as  in  the  case  of  freshly  deposited 
sludge.  For  this  reason  it  becomes  expedient  to  discharge  but 
little  sludge  in  winter,  and  to  make  the  utmost  use  of  the  storage 
capacity  in  the  septic  chambers  of  the  deep  tanks  by  withdrawing 
therefrom  as  much  sludge  as  possible  while  the  weather  is  favor- 
able in  the  summer  and  autumn. 

The  sludge  beds  of  the  Essen-N.  W.  plant  have  an  area  of  1077 
sq.  yds.,  and  in  the  said  period  of  twelve  months  they  drained  a 
volume  of  5500  cbm.,  or  7195  cu.  yds.,  of  liquid  sludge.  This  is  at 
the  rate  of  6.68  cu.  yds.  per  square  yard  of  surface  per  year,  which 
represents  a  depth  of  20.04  ft.  on  the  entire  surface.  The  liquid 
sludge  was  deposited  on  the  beds  to  a  depth  of  from  8  to  10  in. 
at  each  application,  thus  requiring  about  27  or  28  applications 
per  year  in  order  to  drain  the  stated  volume;  and  as  the  average 
period  of  time  required  for  drainage  is  about  6  days  to  each 


SEWAGE  CLARIFICATION  PLANTS  177 

application,  it  follows  that  the  sludge  beds  must  be  in  active  use 
for  an  aggregate  of  6X28  =  168  days  per  year.  This  computa- 
tion shows  that  in  the  climate  of  Essen  ample  time  is  available 
during  the  year  for  the  repeated  fillings,  clearings  and  repairs  of 
the  sludge  beds  after  due  allowance  for  freezing  weather  in 
winter. 

At  all  the  other  plants  of  the  Emscher  Association,  the  experi- 
ence with  the  sewage  sludge  is  similar  to  that  at  Essen-N.  W., 
as  described  above.  It  should  also  be  mentioned  that  at  Reck- 
linghausen,  Holzwickede  and  the  colony  at  the  Count  Schwerin 
Mine,  the  drained  sludge  is  taken  away  by  neighboring  farmers, 
while  at  Essen  and  Bochum,  where  little  agriculture  is  carried  on, 
it  must  be  used  for  filling  depressions  and  low  places.  [The 
populations  tributary  to  the  Recklinghausen,  Holzwickede 
and  mine  colony  plants  are  respectively  30,000,  3200  and  3100, 
while  those  tributary  to  the  Essen-N.  W.  and  Bochum  plants 
are  respectively  60,000  and  145,000.  The  aggregate  dry- 
weather  flow  of  sewage  at  the  first  three  plants  is  about  2,460,000 
U.  S.  gallons  per  day,  while  at  the  last  two  plants  it  is  about 
25,910,000  U.  S.  gallons  per  day  and  contains  much  mine  drainage 
and  ground  water.  The  quantities  of  sludge  produced  annually 
at  each  plant  are  not  given,  but  it  is  obvious  that  only  a  small 
proportion  of  the  drained  sludge  finds  agricultural  utilization. 
Trans.] 

It  is  of  much  interest  to  compare  the  volume  of  the  fresh  sludge, 
as  it  is  deposited  in  the  settling  chamber  or  upper  portion  of  an 
Emscher  tank,  with  that  of  the  decomposed  liquid  sludge  dis- 
charged from  the  bottom  of  the  septic  chamber,  and  also  to 
determine  how  much  of  the  original  volume  is  left  after  the  septic 
sludge  has  been  drained  or  dried  until  it  attains  a  consistency 
like  that  of  moist  earth  which  can  be  cut  and  handled  with  a 
shovel.  Let  us  assume  that  the  fresh  sludge  contains  95  per 
cent,  water.  After  remaining  for  several  weeks  in  the  septic 
chamber,  it  will  contain  only  75  per  cent,  water,  .and  about  one- 
third  of  the  original  quantity  of  organic  matter  will  have  been 
gasified.  In  100  cbm.  of  fresh  sludge  there  will  accordingly  be 
5  cbm.  of  dry  solid  matter,  of  which  65  per  cent,  on  the  average, 
or  3.25  cbm.,  will  be  organic  matter.  Since  one-third  of  this 
latter  substance,  or  1.08  cbm.,  becomes  gasified,  the  remainder 
will  be  (3.25—1.08)  =2.17  cbm.  of  organic  matter.  The  mineral 
matter  amounts  to  5X0.35  =  1.75  cbm.,  and  hence  the  original 
12 


178  SEWAGE  SLUDGE 

volume  of  5  cbm.  of  dry  solid  matter  is  reduced  to  (2.17  +  1.75)  = 
3.92  cbm.,  of  which  55  per  cent,  is  organic  and  45  per  cent,  mineral 
matter.  The  septic  sludge,  however,  contains  75  per  cent, 
water;  hence  with  this  addition  of  water  the  3.92  cbm.  of 
resultant  dry  solid  matter  will  have  a  volume  of  3.92x4  =  15.68 
cbm.  The  original  volume  of  100  cbm.  of  fresh  sludge  has 
thus  reduced  to  a  volume  of  15.7  cbm.  of  the  liquid  septic  sludge 
yielded  by  an  Emscher  tank.  This  represents  a  shrinkage  in 
volume  of  about  84  per  cent. 

Furthermore,  this  liquid  septic  sludge  shrinks  about  40  per 
cent,  in  volume  by  drainage  upon  the  beds  to  a  spadable  con- 
sistency. Its  volume  in  the  aforesaid  case  is  thus  reduced  to 
15.7X0.6  =  9.4  cbm.,  and  hence  we  have  a  total  reduction  in 
volume  of  (100—9.4)  =90.6  cbm.,  or  nearly  91  per  cent.,  of  the 
original  volume  of  100  cbm.  of  freshly  deposited  sludge. 

EXAMINATION   OF   THE   LIQUID   SLUDGE 

The  data  given  in  the  tables  refer  to  average  samples  of  the 
sludge.  In  collecting  samples  for  examination,  a  small  portion 
of  the  liquid  is  taken  at  regular  intervals  during  the  period  of 
discharge  as  it  flows  in  the  open  trough  on  its  way  to  the  sludge 
bed,  and  by  mixing  together  all  these  portions  an  average  sample 
is  obtained.  These  average  samples  are  placed  in  tightly  closed 
jars  and  brought  to  the  laboratory,  where  they  are  usually 
examined  on  the  same  day.  The  examination  generally  em- 
braces the  following  determinations:  1.  The  external  peculiari- 
ties and  smell;  2.  the  amounts  of  contained  water  and  dry* 
matter;  3.  the  proportions  of  organic  and  mineral  substance  in 
the  dry  matter;  4.  amount  of  total  nitrogen  in  the  dry  matter; 
5.  reaction,  alkaline  or  acid;  6.  amount  of  fat  in  the  dry  matter; 

7.  amount  of  gas-making  matter  and  fixed  carbon,  by  coking; 

8.  amount  of  silica,  iron  and  alumina  in  the  ash. 

The  results  of  a  number  of  such  sludge  analyses  are  given  on 
pages  of  Spillner's  paper  on  "The  Drying  of  Sludge."  These 
data  are  now  supplemented  by  the  tables  given  in  the  present 
paper. 

The  sludge  that  is  decomposed  in  the  deep  Emscher  tanks 
is  very  black  in  color,  and  has  the  consistency  of  a  more  or 
less  thick  gruel.  It  is  usually  quite  liquid,  and  flows  easily 
in  a  trough.  In  this  state  it  is  difficult  for  the  unaided  eye  to 


SEW  AGE  CLARIFICATION  PLANTS  179 

recognize  the  nature  of  its  various  components.  Its  reaction  is 
always  slightly  alkaline.  It  has  a  faint  odor  of  india-rubber 
or  tar,  even  in  localities  where  the  liquid  wastes  of  coke  and  gas 
works  are  not  admitted  into  the  sewers.  This  tarry  odor  is  due 
to  the  activity  of  certain  micro-organisms,  and  is  also  found  in 
well-decomposed  river  mud  and  the  sludge  from  other  well- 
ripened  septic  tanks.  It  is  always  faint,  and  can  be  detected  only 
in  the  immediate  vicinity  of  the  mass;  hence  it  cannot  pollute 
the  atmosphere  sufficiently  to  be  regarded  as  a  nuisance  if  the 
sludge  is  properly  decomposed.  Every  Emscher  or  other  septic 
tank,  however,  requires  a  certain  period  of  time  after  being 
placed  in  service  before  its  operation  becomes  satisfactory,  and 
therefore  it  may  happen  that  a  serious  nuisance  will  arise  if  the 
sludge  is  discharged  too  early  upon  the  drainage  beds.  Such  a 
condition  occurred  at  two  of  our  plants,  Recklinghausen  and 
Bochum,  before  we  had  learned  by  experience  how  to  prevent  it; 
but  after  they  had  been  in  operation  a  sufficient  length  of  time 
the  development  of  all  disagreeable  odors  ceased.  If  it  becomes 
necessary  for  any  reason  to  discharge  undecomposed  sludge  dur- 
ing this  ripening  period,  the  material  should  be  treated  like  other 
freshly  deposited  sludge,  such  as  quick  burial  in  the  ground. 
The  large  amount  of  gas  contained  in  Emscher  tank  sludge,  75 
per  cent,  of  which  is  methane  (CH4),  and  therefore  combustible, 
and  25  per  cent,  carbonic  acid  (CO2) ,  has  already  been  mentioned 
in  Spillner's  earlier  paper.  These  gases  also  contain  traces  of 
hydrogen,  nitrogen,  ammonia  and  sulphuretted  hydrogen.  The 
part  played  by  these  gases  in  rendering  the  sludge  mobile  and  in 
facilitating  its  drainage  and  drying  has  also  been  explained. 

The  specific  gravity  of  the  sludge  obviously  varies  with  the 
amount  of  gas  present.  This  is  demonstrated  by  the  fact  that 
from  time  to  time  large  quantities  of  sludge  will  detach  them- 
selves from  the  bottom  of  every  septic  tank  and  rise  to  the  sur- 
face of  the  liquid,  where  they  discharge  their  contents  of  gas  and 
then  sink  again  to  the  bottom.  Sludge  that  is  free  of  gas  has  a 
specific  gravity  of  1.09  to  1.22. 

Details  of  analyses  are  given  in  the  following.  Tables  I,  II  and 
III,  relating  to  the  Recklinghausen,  Essen  and  Bochum  plants. 

In  regard  to  the  analyses  at  Recklinghausen,  it  should  be 
remarked  that  since  the  end  of  1908  this  sludge  cannot  be  con- 
sidered as  normal  Emscher  tank  sludge,  because  the  capacity 
of  the  plant  has  been  greatly  exceeded  by  the  unexpectedly 


180 


SEWAGE  SLUDGE 


rapid  increase  in  the  quantity  of  sewage,  and  hence  the  time 
required  for  a  thorough  decomposition  of  the  sludge  is  no  longer 
available.  The  quality  of  the  sludge,  moreover,  is  different 
from  what  it  was  formerly.  No  bad  results,  however,  have  yet 
appeared,  as  the  change  in  quality  is  manifested  only  by  the 
larger  water  content  of  the  sludge,  and  the  longer  time  required 
for  its  drainage  on  the  beds;  but  it  does  not  become  putrid  in 
drying,  which  is  the  main  thing  to  be  attained.  It  is  intended 
to  relieve  the  tanks  to  some  extent  by  first  passing  the  sewage 
through  a  detritus  chamber  which  will  extract  the  sand  and 
other  heavy  matter.  The  rapid  increase  in  the  quantity  of 
sewage  from  the  other  cities  will  also  affect  the  remaining  plants 
by  reducing  the  time  available  for  the  decomposition  of  the 
sludge. 

Table  I  gives  30  analyses  of  liquid  sludge  from  the  6  Emscher 
tanks  of  the  Recklinghausen  plant,  taken  on  15  different  days 
between  June  14,  1907,  and  September  2,  1910,  three  or  four 
analyses  of  the  same  date  often  relating  to  the  sludge  from 
different  tanks;  the  depth  of  the  tanks  is  not  stated.  The 
essential  figures  are  as  follows: 


TABLE  I 
ANALYSES  OF  EMSCHER  TANK  SLUDGE.     RECKLINGHAUSEN 


Max. 

Min. 

Average 

Water  content  per  cent 

88.3 

75.0 

82.9 

Dry  matter,  per  cent  
Mineral  component  of  dry  matter, 

per  cent  

25.0 

64.4 

11.7 
40.8 

17.1 
54.7 

59  2 

35  6 

45.3 

Nitrogen  component  of  dry  matter 
Fat  component  of  dry  matter,  per 

,  per  cent  
cent  

3.64 
10.79 

1.18 
5.17 

1.74 
6.87 

Table  II  gives  16  analyses  of  liquid  sludge  from  the  9  Emscher 
tanks  of  the  Essen-N.  W.  plant,  taken  on  15  different  days 
between  April  4  and  October  11,  1909;  6  of  these  analyses  refer 
to  either  mixtures  or  averages  from  2  or  3  tanks.  All  these 
tanks  are  9  m.  =  29.5  ft.  deep.  The  essential  figures  are  as 
follows: 


SEWAGE  CLARIFICATION  PLANTS 


181 


TABLE  II 

ANALYSES  OF  EMSCHER  TANK  SLUDGE.     ESSEN 


Max. 

.Min. 

Average 

81.8 

71.3 

75.6 

28.7 

18.2 

24.4 

Miiionl  component  of  dry  matter   per  cent.  .  ;  

53.5 

37.6 

45.1 

Organic  component  of  dry  matter,  per  cent  

62.4 

46.5 

54.9 

1  43 

1  02 

1  22 

Fat  comnonent  of  drv  matter,  ner  cent  .  . 

7.36 

3.44 

4.95 

Table  III  gives  24  analyses  of  liquid  sludge  from  the  18 
Emscher  tanks  of  the  Bochum  plant,  taken  on  11  different  days 
between  February  11,  1909,  and  December  13,  1910.  All  of 
these  analyses  refer  to  single  tanks;  the  depth  of  the  tanks  is  not 
stated.  The  essential  figures  are  as  follows: 

TABLE  III 
ANALYSES  OF  EMSCHER  TANK  SLUDGE.     BOCHUM 


Max. 

Min. 

Average 

Water  content  per  cent  .  

83.9 

70  9 

78  1 

Dry  matter,  per  cent  
Mineral  component  of  dry  matter,  per  cent  
Organic  component  of  dry  matter,  per  cent  
Nitrogen  component  of  dry  matter,  per  cent  
Fat  component  of  dry  matter,  per  cent  

29.1 
71.5 
50.7 
1.50 
12.30 

16.1 
49.3 
28.5 
0.87 
3.53 

21.9 
61.9 
38.1 
1.18 
6.12 

It  has  been  observed  that  the  water  content  of  the  sludge 
depends  in  high  degree  on  the  depth  of  its  surface  (as  determined 
by  sounding  in  the  manner  described  in  the  foregoing)  below 
the  surface  of  the  water  in  the  tank,  and  also  upon  the  age  of  the 
sludge.  If  a  tank  contains  only  a  small  quantity  of  sludge,  the 
presumption  is  that  the  sludge  was  deposited  quite  recently  and 
that  it  will  contain  a  relatively  high  percentage  of  water.  This 
is  always  the  case  in  our  tanks  at  the  end  of  summer,  as  they 
are  operated  so  as  to  discharge  as  much  sludge  as  can  possibly 
be  dried  during  the  warm  season,  and  thus  make  room  in  the 
tanks  for  the  accumulation  of  sludge  during  the  frosty  days 
when  it  cannot  be  discharged  upon  the  drainage  beds.  On  the 
other  hand,  if  the  septic  chambers  are  filled  to  a  high  level  as 


182  SEWAGE  SLUDGE 

will  be  the  case  in  plants  of  sufficient  capacity  to  hold  the  sludge 
accumulations  of  from  2  to  4  months,  the  sludge  will  contain  a 
very  low  percentage  of  water.  The  normal  low  percentage  at 
the  Bochum  and  Essen  plants  is  about  73  per  cent. 

The  easy  separation  of  the  liquid  sludge  into  water  and  a 
spadable  mass  is  explained  not  only  by  the  action  of  the  gases 
already  mentioned,  but  also  by  the  fact  that  the  organic  matter 
has  undergone  an  extensive  decomposition.  Unfortunately  a 
measure  for  this  decomposition  cannot  be  deduced  from  the 
available  analyses,  as  the  data  are  not  sufficiently  complete  to 
admit  of  a  comparison  with  the  composition  of  the  fresh  sludge; 
but  other  investigations  are  now  in  progress  from  which  it  will 
be  possible  to  make  such  a  comparison.  We  can,  however,  form 
a  rough  estimate  of  the  extent  of  the  sludge  destruction  or 
digestion  by  considering  the  volume  of  the  gases  produced. 
Several  measurements  of  this  volume  were  made  at  Essen-N.  W., 
and  it  was  found  that  the  plant  yielded  from  24,700  to  31,800 
cu.  ft.  (700  to  900  cbm.)  of  gases  per  day.  The  weight  of  such  gas 
is  about  (1.6855  Ibs.  per  cubic  yard,  or  0.0624  Ibs.  per  cubic  foot) 
(1  kg.  per  cbm.),  and  hence  from  1540  to  1980  Ibs.  (700  to  900  kg.) 
of  organic  matter  in  the  sludge  were  gasified  every  day.  The 
records  show  that  during  that  period  the  average  daily  produc- 
tion of  decomposed  liquid  sludge  was  18.31  cu.  yds.  (14  cbm.), 
of  which  24  per  cent,  was  dry  matter;  and  as  the  specific  gravity 
of  the  sludge  is  approximately  1.00,  the  daily  yield  of  dry 
matter  was  accordingly  4.39  cu.  yds.  (3.36  cbm.)  or  7407.5  Ibs. 
(3360  kg.),  of  which  about  55  per  cent,  or  4078.5  Ibs.  (1850  kg.) 
is  of  organic  nature.  Let  us  now  assume  that  the  loss  or  de- 
struction of  organic  matter  in  the  sludge  takes  place  exclusively 
by  gasification,  as  we  do  not  yet  know  the  proportion  thereof 
that  is  lost  by  becoming  liquefied;  this  daily  loss  will  then  be 
represented  by  the  aforesaid  weight  of  1540  to  1980  Ibs.  (700  to 
900  kg.),  or  1763.7  Ibs.  (800  kg.)  on  the  average,  of  gas  produced 
every  day  by  the  tanks.  By  adding  this  loss  to  the  aforesaid 
residual  organic  matter  in  the  decomposed  liquid  sludge,  we  will 
have  for  the  daily  quantity  of  organic  matter  that  reaches  the 
tanks:  4078.5  +  1763.7  =  5842.2  Ibs.  (1850  +  800  =  2650  kg.). 
From  this  computation  it  is  seen  that  about  one-third  of  the 
organic  matter  contained  in  the  fresh  sludge  is  lost  or  destroyed 
by  gasification.  [Provided  that  no  liquefaction  occurs.  It 
should  also  be  remembered  that  these  figures  cannot  be  checked 


SEWAGE  CLARIFICATION  PLANTS  183 

by  the  actual  amount  of  organic  matter  in  the  freshly  deposited 
sludge,  which  was  not  ascertained.     Trans.] 

The  foregoing  figures  cannot  be  regarded  as  being  generally 
correct,  as  they  relate  to  only  one  plant  and  a  few  measurements 
of  the  gas  there  evolved.  They  afford,  however,  some  means  of 
estimating  how  much  sludge  is  lost  by  gasification  in  a  properly 
working  Emscher  tank  located  in  our  climate.  Reference  may 
also  be  made  to  the  experiments  of  Favre  and  Spillner  for  deter- 
mining in  another  manner  the  loss  of  sludge  by  decomposition 
in  a  septic  tank,  published  in  Gesundheitsingenieur,  1907,  p.  810 
and  1909,  p.  825,  respectively. 

The  septic  liquid  sludge  is  a  watery  mixture  of  mineral  and 
partly  decomposed  organic  matter.  On  evaporation,  the 
resulting  dry  matter  has  usually  a  gray  color,  but  sometimes 
it  is  brownish-gray.  It  has  little  odor,  and  that  which  is  devel- 
oped when  heated  to  212°  F.  usually  resembles  the  odor  of 
peptone.  In  most  cases  it  contains  few  recognizable  materials, 
but  when  such  are  found  they  are  commonly  bristles,  hair^, 
stems  of  grains,  small  twigs,  scraps  of  leather,  sand,  small  stones, 
and  fragments  of  coal;  bits  of  tinfoil,  card-board,  wood  and 
lime,  and  fish-scales  have  also  been  found  therein  repeatedly. 

The  determination  of  the  total  amount  of  nitrogen  in  the 
dry  sludge  is  made  regularly,  in  view  of  the  utilization  of  this 
material  as  a  fertilizer  at  some  of  the  plants.  The  resulting 
figures  have  exceeded  our  expectations,  the  averages  being 
1.22  per  cent,  at  Essen-N.  W.,  1.39  per  cent,  at  Bochum,  and 
1.57  per  cent,  at  Rechlinghausen.  All  of  the  spadable  sludge 
produced  at  the  latter  plant  has  been  sold  for  fertilizing  pur- 
poses, and  good  results  have  been  attained  therewith. 

Many  determinations  of  the  amount  of  fat  in  the  dry  sludge 
were  made,  but  it  was  found  that  it  was  considerably  less  than 
that  of  the  fresh  sludge  in  other  cities.  Thus  from  16  to  17  per 
cent,  of  fat  was  obtained  from  the  dry  matter  of  the  fresh  sludge 
at  Frankfort,  18  per  cent,  at  Liittich,  15  per  cent,  at  Cassel  and 
14  per  cent,  at  Harburg,  while  the  amount  obtained  from  the 
dried  sludge  of  the  typical  -Emscher  tanks  at  Essen-N.  W.  and 
Bochum  was  only  from  3  to  7  per  cent,  in  most  cases.  The 
difference  must  be  ascribed  to  the  decomposition  attained  in  the 
latter  plants.  Inasmuch  as  the  recovery  of  this  fat  has  never 
proved  profitable  in  other  localities,  it  seems  hopeless  to  attempt 
such  a  process  where  Emscher  tanks  are  used. 


184  SEWAGE  SLUDGE 

Excepting  the  small  proportion  that  is  used  as  fertilizer,  the 
bulk  of  the  drained  sludge  produced  by  the  plants  of  the  Emscher 
Assocation  is  used  at  the  present  time  for  filling  depressions  and 
low  places. 

In  regard  to  the  mineral  matter  of  the  dry  sludge,  a  number 
of  determinations  of  its  principal  components  were  made.  The 
averages  found  at  Essen-N.  W.  are:  SiO2,  63.29  per  cent.;  Fe2O3, 
11.37  per  cent.;  A12O3,  6.56  per  cent.  It  thus  appears  that  the 
mineral  matter  in  the  sludge  consists  chiefly  of  sand. 

EXAMINATIONS  OF  THE  DRAINED  SLUDGE 

The  sludge  is  described  as  "spadable"  when  it  can  be  cut  and 
handled  with  shovels  on  the  drainage  beds  like  moist  earth,  and 
be  loaded  into  the  tram-cars  provided  for  its  transportation  to 
other  localities.  For  the  purpose  of  examination  a  number  of 
samples,  depending  on  the  size  of  the  drainage  bed,  are  collected 
from  different  points  and  are  then  mixed  together;  the  mixture 
is  regarded  as  representing  an  average  sample  of  the  material, 
and  is  then  placed  in  an  air-tight  receptacle  and  taken  to  the 
laboratory.  The  examination  is  made  in  the  same  manner  as 
in  the  case  of  the  liquid  sludge. 

The  surface  of  the  spadable  sludge  has  usually  a  grayish-brown 
color,  while  the  remainder  of  the  mass  is  mostly  black.  Its 
consistence  varies  from  doughy  to  crumbly,  according  to  the 
amount  of  moisture  present,  which  in  turn  depends  on  the  state 
of  the  weather  and  the  length  of  time  allowed  for  drainage.  In 
structure,  the  spadable  sludge  from  Emscher  tanks  is  invariably 
somewhat  spongy.  On  breaking  an  air-dried  sample,  the 
ruptured  surfaces  exhibit  numerous  small  cavities  and  passages 
penetrating  the  entire  mass,  which  were  formed  by  the  bubbles 
of  gas  contained  in  the  liquid  sludge,  and  obviously  facilitate 
both  drainage  and  subsequent  drying  in  high  degree. 

As  the  collection  of  a  fairly  representative  sample  of  a  large 
mass  of  partly  dry  sludge  is  a  matter  of  considerable  difficulty, 
the  examinations  of  spadable  sludge  have  not  been  as  numerous 
as  those  of  the  liquid  sludge.  Table  IV  gives  the  results  of 
13  examinations  of  spadable  sludge  at  the  Essen-N.  W.  plant, 
on  11  different  days  between  April  19  and  August  28,  1909;  and 
with  periods  of  drainage  ranging  from  11  to  3  days.  The 
essential  figures  are  as  follows: 


SEWAGE  CLARIFICATION  PLANTS 


185 


TABLE  IV 
ANALYSES  OF  SPADABLE  EMSCHER  TANK  SLUDGE.      EssEN-N.  W. 


Max. 

Min. 

Average 

59  9 

47  7 

52  3 

52  3 

40  1 

47.7 

Mineral  component  of  dry  matter  per  cent 

54  2 

40  6 

46.6 

Organic  component  of  dry  matter,  per  cent 

59.4 

45.8 

53.4 

Nitrogen  component  of  dry  matter,  per  cent  

1.40 

1.01 

1.17 

Fat  component  of  dry  matter,  per  cent  
Drainage  period,  number  of  days  

3.91 

11 

3.02 
3 

3.39 

7 

The  results  of  5  examinations  of  both  the  liquid  sludge  and  the 
resulting  spadable  sludge  at  the  Essen-N.  W.  plant,  on  the  same 
number  of  days  between  May  25  and  August  7,  1909,  is  given  in 
Table  X  of  Spillner's  paper,  pages  167  and  168. 

The  averages  of  the  results  found  from  the  examinations 
(number  not  stated)  of  spadable  sludge  at  the  Bochum  plant 
during  the  fiscal  year  1910—1911  are: 

Water  content,  63.3  per  cent.;  dry  matter,  36.7  per  cent.; 
mineral  component  of  dry  matter,  64.4  per  cent.;  organic  com- 
ponent of  dry  matter,  35.6  per  cent.;  nitrogen,  1.24  per  cent.; 
fat,  6.91  per  cent. 

Table  V  gives  the  results  of  21  examinations  of  spadable 
sludge  at  the  Recklinghausen  plant,  made  on  9  different  days 
between  May  27,  1908,  and  October  10,  1910.  Two  of  these  were 
made  in  1908  and  the  remainder  in  1910.  The  essential  figures 
are  as  follows: 

TABLE  V 
ANALYSES  OF  SPADABLE  EMSCHER  TANK  SLUDGE.     RECKLINGHAUSEN 


Max. 

Min. 

Average 

Water  content,  per  cent  

73  6 

53  5 

65  2 

Dry  matter,  per  cent  •  
Mineral  component  of  dry  matter,  per  cent  
Organic  component  of  dry  matter,  per  cent  
Nitrogen  component  of  dry  matter,  per  cent  
Fat  component  of  dry  matter,  per  cent  
Drainage  period,  number  of  days  

46.5 
65.4 
58.8 
2.39 
10.39 
22 

26.4 
41.2 
29.8 
0.95 
3.02 
4 

34.8 
•  58.5 
41.5 
1.65 
5.28 
12.5 

It  should  be  noted  that  the  high  average  water  content  (65.2 
per  cent.)  of  the  spadable  sludge  at  Recklinghausen,  together 


186  SEWAGE  SLUDGE 

with  the  long  average  drainage  period  (12.5  days),  is  attributable 
to  the  overworking  of  the  plant  and  the  consequent  lack  of 
thorough  decomposition  of  the  sludge. 

The  examination  of  the  dry  matter  of  the  drained  sludge  is 
made  in  the  same  manner  as  in  the  case  of  the  liquid  sludge. 
The  amount  of  organic  matter  is  generally  somewhat  less  than 
that  in  the  liquid  sludge,  but  it  may  be  remarked  that  in  view 
of  the  large  quantities  of  material  examined,  it  is  very  difficult 
to  obtain  concordant  samples.  Experiments  on  a  small  scale, 
however,  have  shown  that  some  organic  matter  disappears  by 
gasification  during  the  process  of  draining.  A  description 
of  these  latter  experiments  is  given  in  Spillner's  earlier  paper, 
pages  161-164.  Some  instances  of  such  loss  in  drying  are  also 
found  in  Table  X  of  Spillner's  paper  relating  to  the  sludge  of  the 
Essen-N.  W.  plant. 

This  table  likewise  shows  that  the  reduction  in  the  amount  of 
dry  matter  contained  in  the  liquid  sludge,  caused  by  drainage 
on  the  sludge  beds,  ranges  from  0.15  to  0.95  per  cent.  After 
the  spadable  sludge  has  been  removed  to  the  final  dumping 
grounds,  the  process  of  decomposition  continues,  although 
slowly.  No  analyses  have  yet  been  made  in  regard  to  this 
matter,  but  from  the  high  temperatures  (up  to  122°  F.)  that  have 
occasionally  been  observed  in  such  deposits,  it  must  be  con- 
cluded that  further  processes  of  decomposition  are  taking  place 
therein. 

EXAMINATION  OF  THE  LIQUID  DRAWN  FROM  THE  SEPTIC  CHAMBER 

OF  AN  EMSCHER  TANK,  AND  THE  DRAINAGE  WATER 

FROM  THE  SLUDGE  BEDS 

A  knowledge  of  the  composition  of  the  liquid  in  the  septic 
chamber  of  an  Emscher  tank  is  of  interest  to  those  who  operate 
such  plants,  because  this  liquid  is  contained  in  the  sludge  that 
is  discharged  from  the  tank,  and  is  separated  therefrom  in  part 
when  the  sludge  reaches  the  drainage  beds,  and  thence  finds 
its  way  into  the  outfall.  When  a  tank  is  first  put  in  service, 
the  septic  chamber  is  filled  with  the  sewage;  but  as  there  is  no 
current  in  this  chamber,  the  original  volume  of  sewage  soon 
becomes  septic  and  undergoes  thorough  decomposition,  after 
which  it  has  little  odor.  A  renewal  of  the  liquid  by  diffusion 
from  the  sewage  that  flows  through  the  upper  chamber  of  the 
tank,  or  by  the  water  that  is  mixed  with  the  sludge  which  drops 


SEWAGE  CLARIFICATION  PLANTS 


187 


into  the  septic  chamber  through  the  slot  in  the  bottom  of  the 
upper  chamber,  is  usually  a  very  slow  process;  but  when  some  of 
the  accumulated  sludge  is  discharged,  its  volume  is  necessarily 
replaced  with  fresh  sewage  from  the  upper  chamber. 

The  quantity  of  sludge  discharged  at  one  time,  however, 
is  always  a  very  small  fraction  of  the  capacity  of  the  septic 
chamber,  and  the  liquid  therein  is  then  allowed  to  remain  at 
rest  for  several  weeks  as  a  rule.  During  this  time  the  fresh 
sewage  that' replaced  the  volume  of  previously  discharged  sludge 
is  afforded  ample  time  to  become  thoroughly  decomposed.  This 
process  of  decomposition  is  also  accelerated  by  the  constant 
rise  of  gas  bubbles  from  the  sludge  below,  whereby  the  liquid 
contents  of  the  septic  chamber  becomes  thoroughly  intermixed. 

If  a  sample  of  the  liquid  in  the  septic  chamber  is  taken  mid- 
way between  the  floating  scum  at  the  top  of  the  ventilating 
openings  and  the  surface  of  the  dense  sludge  at  the  bottom,  it 
will  be  found  to  be  black  in  color;  and  on  being  allowed  to 
stand,  the  upper  portion  will  gradually  become  clear  while  the 
lower  portion  will  contain  much  sludge.  In  the  tank  a  part 
of  this  sludge  was  being  carried  up  by  the  ascending  gas  bubbles, 
and  another  part  was  in  the  act  of  settling  again  to  the  bottom. 
The  averages  of  the  results  of  a  large  number  of  analyses  of 
such  liquid  is  given  in  the  following  Table  No.  VI: 

TABLE  VI 
ANALYSES  OF  LIQUID  FROM  SLUDGE  CHAMBER  OF  EMSCHER  TANKS 


Name  of  plant 

Reckling- 
hausen 

Boehum 

Essen- 
N.  W. 

Number  of  analyses  made  

17 

6 

15 

Transparency  of  the  liquid  

0.80 

1.97 

0.34 

Reaction  of  the  liquid 

Alkal. 

Alkal. 

Alkal. 

Chlorine,  parts  per  million 

183.0 

993.3 

2193.9 

Residue  after  evaporation,  parts  per  million  

990.7 

2594.1 

4662.2 

Residue  after  ignition,  parts  per  million  

693.7 

2379  .  8 

3961  .  9 

Loss  by  ignition,  parts  per  million  

297.0 

214.3 

700.3 

Suspended  matters,  parts  per  million  

2171.9 

81.8 

5670.2 

Suspended  organic  matter,  parts  per  million  

1044.3 

18.7 

3969  .  9 

Suspended  mineral  matter,  parts  per  million  

1125.6 

63.1 

1700.3 

Nitrates,  parts  per  million  

0 

0 

0 

Nitrites,  parts  per  million  

0 

0 

0 

Total  nitrogen,  parts  per  million  

36.3 

25.4 

70.4 

Nitrogen  as  ammonia  (NHs),  parts  per  million  

27.8 

20.5 

61.4 

Nitrogen  <is  organic  nitrogen    purts  per  million 

8.5 

4.9 

9.0 

Sulphuretted  hydrogen 

Present. 

Present. 

Present. 

188 


SEWAGE  SLUDGE 


On  comparing  these  figures  with  the  corresponding  analyses 
of  the  sewage  in  the  upper  chambers  of  the  Emscher  tanks  at  the 
three  plants  mentioned,  it  will  be  seen  that  the  septic  liquid 
contains  considerably  more  dissolved  matter  than  the  sewage. 

As  already  stated,  this  septic  liquid  is  mixed  with  the  sludge 
that  is  discharged  upon  the  drainage  beds.  These  beds  are 
formed  of  a  layer  of  slag  or  cinders  about  12  in.  deep,  over 
which  another  layer  of  fine-grained  material  is  placed.  This 
upper  layer  absorbs  the  water  that  drains  out  very  slowly 
from  the  sludge,  and  gradually  delivers  it  to  the  underlying 
stratum  of  cinders  from  which  it  escapes  into  the  drain  pipes. 
Both  the  upper  stratum  and  the  lower  one  absorb  much  of  the 
organic  matters  in  the  liquid,  and  these  matters  are  then  trans- 
formed or  mineralzed  by  the  micro-organisms  in  the  inter- 
stices, in  the  same  way  as  in  a  contact  bed  or  sprinkling  filter. 
This  mineralized  matter  is  then  flushed  out  by  the  following 
water,  and  flows  therewith  into  the  underdrains.  The  water 
issuing  from  the  drain  pipes  is  thus  biologically  purified,  and  its 
character  can  be  estimated  from  the  averages  of  the  results  of 
12  analyses  of  samples  of  the  water  taken  from  the  under-drains 
of  the  sludge  beds  of  the  Essen  plant,  given  in  Table  VII,  as 
follows: 

TABLE  VII 

ANALYSES  OF  WATER  FROM  UNDERDRAIN  OF  SLUDGE  BEDS.     ESSEN 


Determinations 

Parts  per 
million 

Determinations 

!  Parts  per 
million 

Transparency 

6  65 

Nitrous  acid  (N  Oj)  .... 

Present 

Residue  after  evaporation 

2674  9 

Nitric  acid  (N'Oj)  

Present 

2311  4 

52  3 

Loss  by  ignition  
Chlorine  
Suspended  matters  

363.5 
529.2 
124.4 
48  8 

Nitrogen  as  ammonia.  .  . 
Organic  nitrogen  
Total    nitrogen    in 
nitrites  and  nitrates  .  .  . 

28.9 
2.1 

21.3 

Suspended  mineral  matter  

75.6 

Putrescibility    

Not  putres. 

These  analyses  relate  in  part  to  waters  derived  from  other 
Emscher  tanks  than  those  which  furnished  the  samples  of  septic 
liquid  cited  in  Table  VI,  and  hence  a  direct  comparison  between 
the  two  results  of  analysis  cannot  be  made.  The  figures,  how- 
ever, indicate  plainly  that  a  biological  purification  has  taken 
place,  as  shown  by  the  presence  of  nitrates  and  nitrites  and  also 


SEWAGE  CLARIFICATION  PLANTS 


189 


by  the  small  quantity  of  organic  nitrogen.  This  drain  water 
can  fairly  be  regarded  as  suitable  for  admission  into  almost  any 
outfall,  and  even  into  such  as  are  sensitive  to  pollution.  It 
accordingly  requires  no  further  treatment,  and  differs  in  this 
respect  greatly  from  the  drainage  waters  of  filter  presses  and 
centrifugal  sludge  driers.  Its  quantity,  moreover,  is  very  small, 
being  only  one  gallon  to  every  10,000  gallons  of  sewage  at 
Essen  N.  W.,  so  that  it  may  be  allowed  to  soak  away  into  the 
ground  at  plants  of  moderate  size. 

YEARLY  COSTS 

The  total  annual  expenses  of  a  sewage  clarification  plant 
consist  of  the  interest  and  sinking  fund  charges  on  the  cost  of  the 
land  and  structures,  the  charges  for  maintenance  and  renewals, 
and  the  usual  outlays  for  supervision,  labor  and  supplies.  The 
figures  for  the  Recklinghausen,  Bochum  and  Essen  N.  W. 
plants,  along  with  some  other  details,  are  exhibited  in  the  fol- 
lowing Table  VIII: 

TABLE  VIII 

COST  OF  OPERATION  AND  MAINTENANCE 


Dry 

Total 

Annual 

Annual 

Name  of  plant 

Tribu- 
tary 
popu- 
lation 

weather 
flow  of 
sewage 
mill.  gals, 
per  day 

Total 

annual 
expenses 

annual 
cost  per 
head  of 
popula- 
tion 

Total 
annual 
cost  per 
million 
gals. 

expense 
for  ope- 
ration 
and 
main- 
tenance 

expense 
for  opera- 
tion and 
mainte- 
nance per 
head 

Recklinghausen.  . 

30,000 

2.38 

$2,355 

$0.0785 

$2.71 

$  750 

$0.0250 

Bochum  

145,000 

13.22          10,188 

0.0703  j      2.11 

3600 

0.0248 

Essen  N.  W  

60,000 

12.69 

6,155 

0.1026 

1.33 

2200 

0.0367 

[It  should  be  noted  that  in  all  of  these  three  plants  neither  the 
sewage  nor  the  sludge  is  pumped,  and  that  no  chemicals  are 
used  to  induce  precipitation  of  sludge  or  disinfection  of  the  efflu- 
ents; also  that  the  effluent  from  the  settling  chambers,  which 
form  the  upper  portions  of  the  Emscher  tanks,  is  not  subjected 
to  any  further  treatment  whatever,  but  flows  directly  into  the 
outfall.  No  figures  are  given  as  to  costs  of  land  and  construc- 
tions; nor  are  the  stated  annual  costs  applicable  to  American 
municipalities  where  materials,  wages  and  labor  are  much  higher 
than  in  Germany. — Trans.] 


190  SEWAGE  SLUDGE 

GENERAL  RESULTS 

If  all  connections  between  the  sewers  and  privy-vaults  or  cess- 
pools are  abolished,  the  water-carried  sewage  will  reach  the 
disposal  plant  in  a  fresh  condition.  The  temperature  of  the 
sewage  in  winter  is  high  enough  to  prevent  its  freezing  in  the 
plant.  [The  lowest  temperature  of  the  air  at  these  plants  during 
the  14  months  from  January  1,  1909,  to  March  1,  1910,  was  about 
14°  F.,  and  prevailed  for  only  two  or  three  consecutive  days; 
the  temperature  diagrams  given  in  the  paper  show  that  the 
temperature  of  the  air  was  below  the  freezing-point  (32°  F.) 
on  from  12  to  30  days  in  the  aggregate  during  this  period. — 
Trans.] 

If  the  sewage  is  not  septic  when  it  reaches  the  plant,  it  will  not 
become  septic  during  its  passage  through  the  upper  settling 
chambers  of  the  Emscher  tanks.  [The  cylindrical  tanks  are 
combined  into  groups  of  three  at  the  plants  mentioned,  thus 
making  the  length  of  each  continuous  settling  chamber  about 
100  ft.,  and  the  time  allowed  for  the  sewage  to  traverse  this 
distance  ranges  from  30  to  56  minutes  on  the  average  in  dry 
weather.  When  the  volume  of  sewage  is  increased  by  rainfall, 
the  time  of  passage  is  reduced. — Trans.] 

Tests  for  putrescibility  of  mixtures  of  the  effluent  with  the 
unpolluted  water  of  the  several  small  streams  into  which  the 
clarified  sewage  is  discharged,  show  that  offensive  odors  are  not 
developed  in  a  mixture  of  equal  parts  of  effluent  and  clean  water, 
and  sometimes  not  in  a  mixture  of  two  parts  of  effluent  to  one  of 
clean  water,  even  when  kept  standing  in  an  incubator.  The 
putrefaction  of  the  stream  is  therefore  not  to  be  apprehended. 

If  the  time  of  flow  through  the  aforesaid  settling  chamber  is 
not  less  than  45  minutes,  about  95  per  cent,  on  the  average  of 
the  entire  volume  of  sedimentable  matter  in  the  sewage  will  be 
deposited  in  the  septic  chambers  underneath.  The  effluent 
will  accordingly  contain  not  more  than  one  volume  of  sediment- 
able  matter  in  2000  equal  volumes  of  the  liquid.  [The  term 
"sedimentable  matter"  applies  to  the  sludge  or  sediment  which 
settles  in  the  course  of  2  hours  into  the  lower  part  of  a  recep- 
tacle filled  with  crude  sewage  and  left  undisturbed. — Trans.] 

Coal  dust  requires  a  much  longer  time  for  settlement  than 
the  sludge  of  domestic  sewage;  in  experiments  with  fine  coal 
dust  it  was  found  that  from  4  to  8  hours  were  necessary  for  its 


SEWAGE  CLARIFICATION  PLANTS  191 

sedimentation.  It  is  therefore  important  to  exclude  from 
the  sewers  all  waters  that  have  been  used  for  washing  coal  and 
coke  and  have  not  undergone  long  sedimentation. 

When  the  volume  of  sewage  is  largely  increased  by  storm 
water,  no  appreciable  quantity  of  sludge  can  be  flushed  out 
from  an  Emscher  tank,  because  the  sludge  does  not  accumulate 
in  the  settling  chamber  through  which  the  sewage  flows,  but 
drops  at  once  into  the  septic  chamber  below  where  it  remains 
undisturbed  by  the  current  in  the  upper  chamber. 

The  volume  of  the  fresh  sludge  deposited  in  an  Emscher  tank 
is  reduced  about  85  per  cent,  by  the  process  of  decomposition  in 
the  septic  chamber.  From  25  to  30  per  cent,  of  the  dry  matter 
contained  in  the  fresh  sludge  will  be  destroyed  by  gasification. 
The  gases  thus  produced  consist  mostly  of  methane  (CH4)  and 
carbonic  acid  (CO2).  Offensive  odors  are  not  developed  in  the 
process  of  gasification.  The  rest  of  the  reduction  in  the  volume 
of  the  sludge  is  principally  due  to  the  diminution  of  its  content 
of  wrater.  The  proportion  lost  by  liquefaction  is  as  yet  unknown. 

The  drying  or  draining  of  the  sludge  to  a  spadable  consist- 
ency upon  suitable  beds  is  accomplished  in  6  days  on  the  average. 
A  depth  of  from  8  to  10  in.  of  liquid  sludge  is  deposited  on  the 
beds  each  time  they  are  used.  In  the  course  of  the  drying 
process  the  volume  of  the  sludge  is  reduced  about  25  per  cent. 
During  a  year  an  aggregate  depth  of  20  ft.  of  liquid  sludge  is 
applied  to  each  bed.  After  the  spadable  sludge  reaches  the 
dumping  ground,  it  is  gradually  transformed  into  an  earthy 
substance.  The  drainage  water  from  the  sludge  beds  is  bio- 
logically purified  in  passing  through  them. 

The  cost  of  clarifying  sewage  with  Emscher  tanks  is  small, 
as  seen  from  the  preceding  table. 


SLUDGE  TREATMENT  IN  THE 
UNITED  STATES 

BY 

KENNETH  ALLEN,  M.  AM.  SOC.  C.  E. 


13 


SLUDGE  TREATMENT  IN  THE 
UNITED  STATES 


I.  AMERICAN  SEWAGE 

In  applying  data  of  sewage  purification  the  character  of  the 
sewage  itself  is  of  first  importance:  in  particular,  its  composition 
and  age.  It  should  be  borne  in  mind  that  results  obtained  under 
European  conditions,  where  a  water  consumption  of  about 
40  gallons  per  capita  daily  is  a  usual  amount,  may  be  quite 
different  under  American  conditions,  where  a  water  consumption 
of  100  or  even  125  gallons  per  capita  daily  is  not  uncommon. 
So,  too,  a  distinction  is  necessary  between  sewages  from  com- 
bined and  separate  systems,  the  former  being  greatly  diluted 
and  increased  in  volume  during  rainy  weather,  besides  bringing 
with  it  much  grit  from  the  street  surfaces.  Trade  wastes,  when 
produced  in  excessive  amounts,  often  constitute  a  special 
problem,  either  when  taken  in  combination  with  the  ordinary 
sewage  or  when  treated  independently;  but  ordinarily  their 
influence  is  not  sufficient  to  determine  the  method  of  puri- 
fication to  be  adopted. 

The  amount  of  the  impurities  to  be  dealt  with  in  the  case  of 
dry-weather  or  domestic  sewage  depends  on  the  population 
served  rather  than  on  the  volume  of  liquid,  so  that  in  questions 
relating  to  sludge  treatment  populations  are  generally  preferable 
as  a  basis  of  computation  rather  than  volumes  of  liquid. 

Table  I  gives  the  analyses  of  the  sewage  of  several.  American 
cities,  and  Table  II  gives  the  amount  of  the  suspended  solids 
and  the  part  of  this  which  is  organic  or  liable  to  cause  offensive 
conditions  through  putrefaction — figures  of  first  importance 
in  question  of  sewage  clarification  and  sludge  treatment. 

195 


196 


SEWAGE  SLUDGE 


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SLUDGE  TREATMENT  IN  THE  UNITED  STATES    197 

TABLE  II 

SUSPENDED  MATTER  IN  THE  SEWAGE  OF  SEVERAL  AMERICAN  CITIES 
a.  Parts  per  Million 


Place 

Total 

Organic     Mineral 

Authority 

Boston,  Mass  

135 

91               44 

Kinnicutt,  Winslow  &  Pratt. 

Chicago,  111  

143 

80               63 

Eng.  News,  Mar.  31,  1910. 

Columbus,  O  

215 

81             134 

Geo.  A.  Johnson. 

Lawrence,  Mass  

149 

113               36 

Kinnicutt,  Winslow  &  Pratt. 

Mass  —  Small  towns 

94.6 

78.1           16.5 

Kinnicutt,  Winslow  &  Pratt. 

Small  cities  

180.6 

46.4          31.2 

Kinnicutt,  Winslow  &  Pratt. 

Paterson,  N.  J  

45  to  641 

George  C.  Whipple. 

Philadelphia,  Pa  

204 

142               62 

Geo.  S.  Webster. 

Plainfield,  N.  J  134 

Andrew  Gavet. 

Providence,  R.  I  397 

343.5           53.5 

Kinnicutt,  Winslow  &  Pratt. 

Waterbury,  Conn  165 

115               50 

Eng.  News,  June  3,  1909. 

Worcester,  Mass  255  .  8 

177.8          78.0 

Kinnicutt,  Winslow  &  Pratt. 

b.  Gra  ms  per  C  apita. 

Chicago,  111  

166 

93               73 

Eng.  News,  Mar  31,  1910. 

Columbus,  O  

98 

47               51 

Geo.  A.  Johnson. 

Mass.  —  Small  cities  

53 

44                  9 

Kinnicutt,  Winslow  &  Pratt. 

Mass.  —  Separate  systems.  .           49 

38               11 

Geo.  A.  Johnson. 

Mass.  —  Combined  and  mfg  .         145 

76               69 

Geo.  A.  Johnson. 

United  States  93 

40               53 

Geo.  W.  Fuller. 

Although  the  composition  of  sewage  varies  greatly  in  different 
cities  the  following  analyses  may  be  taken  as  fairly  representing 
ordinary  American  conditions. 


198 


SEWAGE  SLUDGE 

TABLE  III 
COMPOSITION  OF  TYPICAL  AMERICAN  SEWAGE 

In  Grams  per  Capita  Daily1 
1.  According  to  George  C.  Whipple2 


• 

Domestic 
sewage 

Sewage  of  manufacturing 
cities 

Total  solids  
Organic  matter  
Mineral  matter  ... 

170 
70 
100 

220  to  500 
100  to  200 
120  to  300 

Chlorine 

20 

25  to    50 

Nitrogen  
Albuminoid  ammonia.  .  .  . 
Free  ammonia  
Fats  

11 
1.7 

7 
20 

13  to    15 
2  to      4 
5  to    10 
20  to    50 

2.  According  to  George  W.  Fuller 


Oxygen  consumed  .... 
Nitrogen  as 

.  .  2  minutes'  boiling  .  .  .  . 
5  minutes'  boiling  .  .  .  , 
Free  ammonia 

.      15.0 
.      22.0 
7.0 

Albuminoid  ammonia 
Organic  
Total 

..        2.5 

.       8.0 
15.0 

Chlorine 

.      19.0 

Fats                    

,  .      19.0 

Dissolved  matter  

.  .    Mineral  
Organic  and  volatile  . 
Total 

.      99.0 
..      37.0 
.    136.0 

Suspended  matter.  .  .  . 

/ 
Total  solids  

.  .    Mineral  
Organic  and  volatile  . 
Total  
.  .    Mineral  

.      53.0 
.  .      40.0 
.  .      93.0 
.    152.0 

Organic  and  volatile  . 
Total  .  . 

.      77.0 
.    229.0 

1  To  convert  to  ounces  per  capita  multiply  by  0.0327. 
To  convert  to  grains  per  capita  multiply  by  15.432. 

If  the  volume  of  sewage  is  taken  as  100  gallons  per  capita  daily: 
To  convert  to  grains  per  gallon    multiply  by  0.1543. 
To  convert  to  parts  per  million  multiply  by  2.6417. 

2  Report  on  Sewage  Disposal,  Paterson,  N.  J.,  1906. 

3  Technology  Quaterly,  June,  1903,  p.  141. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    199 

3.  According  to  E.  B.  Phelps  l 


In 
solution 


In 

suspension 


Total 


Mineral  and  ash 

Organic  and  volatile 

Total  residue  on  evaporation  .  . 


114 
76 

190 


38 
76 

114 


152 
152 

304 


The  organic  matter  is  composed  as  follows: 

Total  carbon 76 . 0 

Total  nitrogen 5.7 

Total  H,  O,  S,  P,  etc 70.3 

152.0 

Separating  the   nitrogenous   from   the   carbonaceous   matter 
there  results: 

Nitrogenous  matter: 

Nitrogen 5.7 

Carbon 28 . 5 

H,  O,  S,  P,  etc 22.8 

57.0 

Fats,  etc.: 

Carbon 13.3 

H  and  O 5.7 

19.0 
Carbohydrates : 

Carbon 34.2 

H,  O,  etc 41.8 

7(>.0 
Total..  .    152.0 


II.    DETRITUS  FORM  GRIT  CHAMBERS 

Boston,  Mass. — At  the  sewage  experiment  station  of  the 
Massachusetts  Institute  of  Technology,  the  sewage  was  pumped 
by  a  small  duplex  pump  from  a  large  sewer  carrying  the  combined 
sewage  of  350,000  persons.  A  small  grit  chamber  was  formed  of 

1  Deduced  from  Water  Supply  and  Irrigation  Paper,  No.  185.  Table,  p.  15,  Assuming 
100  Gals.  Per  Cap. 


200 


SEWAGE  SLUDGE 


a  cast  iron  cylinder  19  in.  in  diameter  and  16  in.  deep,  containing 
a  screen  with  1/2-in.  meshes.  The  velocity  was  reduced  in  this 
to  0.04  ft.  per  second,  thus  making  the  time  of  passage  through 
the  chamber  about  45  seconds. 

The  detritus  removed  from  this  chamber  amounted  to  0.65 
cu.  yds.  per  million  gallons  of  sewage.  It  contained  27  per  cent, 
moisture  and  but  6.65  per  cent,  organic  matter,  and  were  quite 
inoffensive  when  spread  on  the  land  adjoining  the  station.1 
Analyses  of  samples  taken  from  March  26;  1904,  to  June  1,  1905, 
averaged  as  follows:2 


Clean 

Fine  dry   detritus 

Wet 

detritus 

etc. 

Total 

Loss  on 
ignition 

Organic  N 

Oxygen 
consumed 

Pounds  per  mil- 

1600 

430 

190 

970 

106 

2.2 

1.7 

lion  gallons  sew- 

age. 

Parts  per  million 

190 

52 

23 

117 

13 

.26 

.2 

parts  of  sewage. 

Near  the  Moon  Island  outlet  of  the  Boston  Main  Drainage 
system  the  outfall  sewer  is  enlarged  to  form  two  conduits  8  ft. 
wide,  16  ft.  high  and  about  1/4  mile  in  length,  in  which  the 
heavier  solids  deposit.  The  depth  of  sewage  in  these  sewers  of 
deposit  is  designed  to  be  from  8  to  10  ft.  The  sludge  is  pushed 
toward  one  end  where  it  is  drawn  off  by  a  12-in.  pipe  to  a  sludge 
tank  50  ft.XlO  ft.Xl5  ft.  in  size,  having  a  capacity  of  150  cu. 
yds.  From  this  tank  it  is  taken  by  a  scow,  wrhich  is  towed  about 
20  miles  to  sea,  and  dumped. 

In  the  year  ending  February  1,  1910,  with  an  average  flow  of 
82,378,000  gallons  per  day,  8773  cu.  yds.  of  sludge  was  deposited 
in  these  sewers,  or  0.29  cu.  yds.  per  million  gallons  of  sewage,  in 
addition  to  whatever  was  subsequently  deposited  in  the  storage 
tanks  on  Moon  Island. 

Worcester,  Mass. — The  sewage,  which,  in  1910,  averaged  14.57 
million  gallons  per  day,  or  107.2  gallons  per  capita,  passed 
through  one  of  two  grit  chambers  40  ft.  X  10  ft.  in  plan  and  9  ft. 
deep  in  about  1.8  minutes.  The  mean  velocity  was,  therefore, 
0.4  ft.  per  second. 

1  Experiments   on   the  Purification   of   Boston   Sewage,    Winslow   and  Phelps.     WTater 
Supply  and  Irrigation  Paper  No.  185. 

2  Kinnicutt,  Winslow  and  Pratt. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    201 


According  to  Mr.  Matthew  Gault,  superintendent  of  sewers, 

>.")  cu.  yds.  of  heavy  grit,  about  half  water  and  weighing  18,15)7 
Ibs.  per  cubic  yard,  were  removed,  representing  4.0  per  cent,  of  the 
total  suspended  matter  in  the  sewage.  The  effluent  contained 
276  parts  per  million  of  suspended  matter.  The  material  re- 
moved amounted  to  0.11  cu.  yds.  per  million  gallons  of  sewage. 
The  cost  of  removal  from  the  grit  chambers  and  placing  it  in 
carts  (shoveling  3  times)  was  33  1/3  cts.  per  cubic  yard,  and  the 
cost  of  hauling  about  600  ft.  and  dumping,  50  cts.  per  cubic  yard, 
making  the  total  cost  of  disposal  83  1/3  cts.  per  cubic  yard  or 
91/4  cts.  per  million  gallons  of  sewage. 

Columbus,  Ohio.  —  The  first  grit  chamber  used  at  the  experi- 
mental station  was  40  ft.XS  ft.XT  1/2  ft.  deep.  This  was 
subsequently  changed  to  39.5  ft.  X5  ft.X2.5  ft.  in  depth,  with  a 
bottom  baffle  a  foot  high,  2  ft.  from  the  inlet  and  a  surface  baffle 
extending  to  about  6  in.  from  the  bottom,  3  ft.  from  the  inlet. 

In  the  former  the  average  velocity  was  0.518  ft.  per  minute 
(2.61  mm.  per  second)  and  the  period  of  flow  1.3  hours;  in  the 
latter  the  average  velocity  was  2.28  ft.  per  minute  (11.39  mm. 
per  second)  and'  the  period  of  flow  0.29  hour. 

The  results  obtained  in  the  two  chambers  were  as  follows: 

TABLE  IV 


Original  grit 

Remodelled  grit 

chamber 

chamber 

Per   cent,    suspended   matter   in      Total  

34 

22 

applied      sewage      which      was       i  Volatile  .... 

30 

18 

removed  in  grit  chamber  i  Mineral  .... 

35 

24 

Cubic  yards  wet  sludge  per  million  gallons  

2.55 

1.76 

Per  cent,  moisture  in  sludge,  average  

87 

83 

Per  cent,  volatile  matter  in  dry  solids  

52 

46 

COMPOSITION  OF  GRIT  CHAMBER  SLUDGE 

Weight,  per  cubic  yard 1825  Ibs.. 

Specific  gravity 1 .081. 

Water 82 .4   per  cent. 

Soilds 17.6    per  cent. 

Volatile  matter 7.9    per  cent. 

Nitrogen 0 . 22  per  cent. 

Fats 1 .22  per  cent 

It  is  difficult  to  reach  a  comparison  of  the  above  results  of 
rapid  sedimentation  in  grit  chambers.  The  quantity  and  quality 
of  the  material  removed  depends  upon  various  conditions,  such 


202  SEWAGE  SLUDGE 

as  the  admission  of  storm  water,  the  character  of  street  surfaces, 
the  use  and  efficiency  of  catch  basins  for  the  interception  of  grit, 
the  velocity,  depth  and  time  of  passage  through  the  grit  chamber 
and  the  per  cent,  of  moisture  in  the  detritus.  With  strictly 
separate  systems  and  domestic  sewage  the  amount  would  be  so 
small  as  to  be  an  insignificant  factor  in  questions  of  disposal, 
while  in  combined  systems,  where  the  volume  may  approximate 
a  cubic  yard  per  million  gallons  of  sewage,  the  amount  of  putres- 
cible  matter  is  usually  so  small  that  the  detritus  may  often  be 
used  for  filling  in  land. 

Mr.  Emil  Kuichling1  mentions  the  results  obtained  in  various 
foreign  cities,  at  the  Boston  experiment  station  of  the  Massa- 
chusetts Institute  of  Technology  and  at  the  Dorchester  pumping 
station  in  that  city  [0.31  cu.  yds.  per  million  gallons]  and  obtains 
an  average  of  0.4  cu.  yds.  per  million  gallons  with  a  specific  gravity 
varying  from  1.52  to  1.87  and  a  water  content  of  27  per  cent. 
Under  these  assumptions  the  dried  suspended  matter  removed  by 
grit  chambers  is  about  835  Ibs.  per  million  gallons,  or  100  parts 
by  weight  per  million. 

Waterbury,  Conn.2 — The  water  supply  at  Waterbury  is  139 
gallons  per  capita,  and  the  sewers  are  on  the  combined  system. 
The  grit  chamber  was  cleaned  frequently  so  that  no  gases  were 
formed  in  the  detritus. 


DATA 

Cu.  yds.  removed  per  million  gallons  of  sewage  ...  1 .40 

Tons  removed  per  million  gallons  of  sewage  .....  1 . 12 

Specific  gravity 1 . 05 

Moisture : 88 . 3    per  cent. 

Solids 11.7    per  cent. 

Mineral  matter 5.9    per  cent. 

Fats 0.78  per  cent. 

Nitrogen 0 . 22 :  per  cent. 

III.  SCREENINGS 

A  sharp  distinction  should  be  made  between  coarse  bar  screens 
intended  primarily  to  intercept  sticks  of  wood,  orange  and  lemon 
peels,  rags,  etc.,  and  the  fine  screens  having  clear  openings  of  3/8 
in.  or  less,  which  have  been  introduced  in  increasing  numbers, 

1  Notes  on  Sewage  Disposal,  Rochester,  1910. 

2  Eng.  News,  Vol.  LXI,  p.  596. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    203 

especially  in  Germany,  to  remove  as  much  of  the  matter  in  sus- 
pension, including  fecal  matter,  as  practicable. 

The  former  type  is  customary  in  all  systems  where  the  sewage 
has  to  be  pumped,  but  it  has  not  usually  been  considered  worth 


FIG.  37.— Bar  screen,  Dorchester  Pumping  Station,  Boston. 
(Courtesy  of  F.  L.  Sanborn,  Executive  Engineer,  Boston  Main  Drainage.) 

while  to  measure  the  detritus  removed.     The  following  figures 
are,  however,  available. 

Boston,  Dorchester  Pumping  Station. — Here  the  combined 
sewage,  amounting  in  1909  to  96,373,000  gallons  per  day,  passes 
through  screen  cages  7  ft.  X3  ft.X7  ft.  high,  having  3/4  in. 


204  SEWAGE  SLUDGE 

vertical  bars  on  3  sides  spaced  1  in.  apart  and  a  floor  upon  which 
the  screenings  fall  on  raising  the  cage,  and  from  which  they  are 
removed  by  hand  and  pressed  to  remove  the  excess  moisture. 

In  the  year  ending  February  1,  1910,  573  1/4  tons  of  wet  filth 
were  removed,  or  38.1  Ibs.  per  million  gallons.  The  cost  of  labor 
at  the  screens  was  0.313  cts.  per  million  gallons. 

Boston  Metropolitan  System. — The  screens^  at  the  several 
pumping  stations  of  the  Metropolitan  Sewerage  System  are  in 
type  similar  to  those  at  the  Dorchester  Pumping  Station  of  the 
Main  Drainage  works.  The  screens  are  composed  of  3/4-in. 
round  bars,  1  1/2  in.  center  to  center  and  a  similar  series  stag- 
gered in  front  of  these,  providing  an  effective,  clear  space  of 
about  1/8  in.1 

The  following  table  gives  the  amount  of  wet  screenings  removed 
from  the  North  and  South  Metropolitan  Systems  from  the  be- 
ginning of  operation,  amounting  to  0.10  and  0.16  cu.  yds.  per 
million  gallons,  respectively,  during  the  year  1910,  or  5  and 
10  cu.  yds.  annually  per  thousand  population.  The  sewage  is 
partly  separate  and  partly  combined  in  each  system. 

The  material  removed  from  the  screens  at  the  Charlestown  and 
East  Boston  Pumping  Stations  June  10,  1898,  had  the  following 
composition:2 

Paper 55  per  cent. 

Rags 25  per  cent. 

Hair 5  per  cent. 

Fecal  matter  and  grease 5  per  cent. 

Refuse  from  slaughter  houses 4  per  cent. 

Conglomerate  matter 6  per  cent. 

100  per  cent. 

Columbus  Experimental  Plant. — Two  vertical  removable  screens 
were  used,  consisting  of  a  diamond  mesh  of  No.  12  wire,  the 
first  having  1/2-in.  and  the  second  3/8-in.  openings.  These 
removed  36  of  the  215  parts  per  million  of  suspended  matter  con- 
tained in  the  sewage,  or  0.17  cu.  yds.  weighing  300  Ibs.  per  million 
gallons.  The  weight  per  cubic  yard  was  therefore  about  1765  Ibs. 

This  amount  would  undoubtedly  have  been  considerably 
greater,  but  for  the  fact  that  the  sewage  treated  was  not  drawn 
from  the  invert  of  the  trunk  sewer  and  so  did  not  contain  all 
the  grit  and  coarse  heavy  matter  moving  along  the  bottom. 

1  W.  M.  Brown,  chief  engineer. 

2  Metropolitan  Sewage  Com'rs,  1899,  p.  25. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    205 


& 


w 


I 


?;  § 


^ 


i  s 


S     8 

I  * 


I  S, 


206  SEWAGE  SLUDGE 

The  liquid,  moreover,  had  previously  passed  through  a  1/2-in. 
screen  and  was  then  pumped,  by  which  much  coarse  material  was 
removed  and  fecal  matter  broken  up. 

Philadelphia  Experimental  Plant. — This  was  supplied  with 
separate  system  sewage  from  a  4  ft.  7  in.  intercepting  sewer 
through  an  8-in.  pipe,  which  entered  the  sewer  15  in.  above  the 
invert.  The  sewage  was  then  pumped  through  413  ft.  of  4-in 
pipe  to  a  point  near  the  testing  station,  from  which  it  was  drawn 
by  gravity.  Some  of  the  heavy  solids  were  probably  excluded 
at  the  start,  and  the  soft  fecal  matter  must  have  been  disin- 
tegrated by  pumping,  as  at  Columbus. 

The  sewage  was  delivered  through  24  1/4-in.  nozzles  upon 
the  conical  surface  of  a  screen  having  32  meshes  per  inch  (clear 
openings  0.5  mm.  square). 

The  amount  of  suspended  matter  varied  considerably,  owing 
to  the  admission  of  trade  wastes,  but  averaged  from  September, 
1909,  to  April,  1910,  inclusive,  about  200  parts  per  million,  7/10 
of  which  was  volatile.  Of  this,  63  parts  per  million,  or  33.5  per 
cent.,  were  removed  by  the  fine  screen,  equivalent  to  560  Ibs.  of 
dry  solids  per  million  gallons  of  sewage. 

The  effect  of  screening  on  subsequent  treatment  was  found 
to  be: 

1.  A  more  uniform  sewage  by  eliminating  in  part  the  irregu- 
larities due  to  trade  wastes. 

2.  A  reduction  in  the  sludge  subsequently  treated. 

3.  An  increase  of  moisture  in  the  sludge  subsequently  treated. 

4.  A  finer  subsequent  sludge  and  one  more  readily  pumped. 

5.  An  entire  absence  during  9  months  of  clogging  in  sprinkler 
nozzles  using  settled  screened  sewage. 

Reading,  Pa. — The  only  important  example  as  yet  of  fine 
screening  on  a  working  scale  in  the  United  States  is  that  at 
Reading,  Pa.,  with  an  apparatus  devised  by  Mr.  O.  M.  Weand. 
This  consists  of  a  horizontal  cylindrical  framework  6  ft.  in  diam. 
X12  ft.  long,  which  is  supported  on  rollers  and  is  rotated  by 
means  of  a  circumferential  gear  at  a  rate  of  8  revolutions  per 
minute.  The  cylindrical  framework  is  covered  with  wire  cloth 
of  monel  metal  having  from  30  to  36  meshes  to  the  linear  inch, 
which  is,  in  turn,  supported  by  a  screen  of  No.  12  copper  wire 
with  5 /8-in.  meshes. 

The  sewage  enters  at  one  end  and  is  distributed  by  flowing  over 
a  weir  placed  in  the  first  half  of  the  cylinder  parallel  to  the  axis. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    207 


It  then  flows  through  the  bottom  of  the  screen  and  is  conveyed 
away,  leaving  the  sludge  and  other  coarse  material  inside. 

By  the  rotation  of  the  screen  the  sludge  is  worked  forward 
by  a  narrow  spiral  plate,  which  projects  from  the  inner  surface 
until  it  reaches  the  further  end,  where  it  is  lifted  by  a  series  of 
short  radial  buckets  attached  to  the  perimeter.  On  reaching  a 


certain  elevation  the  sludge  slips  off  upon  a  sloping  trough, 
which  delivers  it  through  the  end  of  the  cylinder  and  drops  it  into 
a  receptacle  from  which  it  is  raised  and  transported  by  suitable 
conveying  machinery  to  an  elevated  sludge  tank. 

The  sludge  that  collects  on  the  screen  is  washed  off  by  12  1/16- 
in.  water  jets  in  each  of  two  horizontal  pipes  placed  just  outside 


208  SEWAGE  SLUDGE 

the  screen — one  on  each  side.  Each  pipe  is  moved  back  and 
forth  longitudinally  by  a  toggle  joint  at  one  end  so  that  the 
entire  surface  of  the  screen  can  be  freed  of  detritus.  Screened 
sewage  has  been  used  in  place  of  water  for  this  purpose  satis- 
factorily, but  the  jets  clog  rapidly  with  the  unscreened  liquid. 
Hair,  lint  and  other  fibers  are  not  so  readily  removed,  however, 
and  this  difficulty,  inherent  in  any  fine  wire  mesh,  would  prob- 
ably cause  trouble  in  its  use  with  some  classes  of  sewage. 

According  to  Mr.  C.  B.  Ulrich,  City  Engineer  of  Reading,  the 
volume  of  sewage  handled  is  about  5  million  gallons  daily  from 
40,000  persons.  It  contains  125  or  130  parts  per  million  of 
suspended  solids.  The  screenings  amount  to  1.15  cu.  yds.  per 
million  gallons,  and  have  the  following  general  composition: 

Wet  After  Centrifuging 

Moisture 89 . 5  per  cent.  73. 0  per  cent. 

Mineral  matter 2.8  per  cent.  7.4  per  cent. 

Volatile  matter 7 . 7  per  cent.  19. 6  per  cent. 


lOO.Opercent.      100.0  per  cent. 

The  weight  of  the  wet  screenings  is  stated  by  Mr.  Weand, 
who  had,  until  recently,  the  contract  for  operation,  to  be  about 
63  Ibs.  per  cubic  foot  and  the  cost  of  operation,  including 
about  5  h.  p.  of  steam  power  required  for  rotating  the  screen 
and  driving  a  centrifugal  separator,  to  be  $1.00  per  million 
gallons  when  taking  4  million  gallons  per  day. 

One  difficulty,  due  to  the  fine  mesh  employed,  is  the  frequent 
stoppage  for  repairs.  This  amounted  to  1500  hours,  equivalent  to 
63  days,  in  1910,  and  an  equivalent  of  77  days  in  1909.  Screens 
of  this  type  should  always,  therefore,  be  in  duplicate. 

Another  objection  in  some  cases  would  be  the  loss  of  several 
feet  head  caused  by  the  drop  in  the  sewage  through  the  screen.1 

A  high  percentage  of  moisture  in  the  screenings  is  probably 
inevitable  with  fine  meshes  and  domestic  sewage.  In  his  annual 
report  for  1910  Mr.  Ulrich  says:  "The  criterion  for  screening 
efficiency  should  be  the  thoroughness  with  which  the  larger 
suspended  matters  are  removed  and  not  only  mere  bulk  of  re- 
moved matters.  All  materials  large  enough  to  clog  sprinkler- 
nozzles  should  be  screened  out,  but  it  is  more  economical  and 
just  as  satisfactory  to  remove  finer  solids  by  sedimentation." 

1  By  a  recently  devised  modification  of  design  Mr.  Weand  hopes  to  save  the  greater 
part  of  this  lost  head. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    209 


It  would  appear,  too,  that  with  the  constant  agitation  of  the 
fecal  matter  by  the  wash  water  and  the  rotation  of  the  screen, 
much  of  it  will  have  become  so  finely  broken  up  as  to  pass  through 
with  the  sewage. 

But  in  spite  of  these  disqualifications,  the  local  authorities  are 
satisfied,  on  the  whole,  with  the  results  secured  and  have  recom- 
mended the  installation  of  a  second  unit. 

Screens  of  this  general  type  are,  or  soon  will  be,  installed  at 
Atlanta,  Ga.,  Brockton,  Mass.,  New  Brunswick,  N.  J.  and  Bal- 
timore, Md. 

Providence,  R.  I. — Here  the  screen  bars  are  of  wood  1  in.  X 
10  in.  in  section,  spaced  3/4  in.  apart,  forming  an  inclined  rack 
69  ft.  in  length,  inclined  17°  to  the  vertical,  through  which  the 
sewage — about  19.8  million  gallons  per  day — flows,  with  a  depth 
of  from  2  to  5  1/2  ft.,  averaging  about  3  ft.  The  screen  is  cleaned 
continuously  by  hand  with  rakes.  The  screenings,  consisting 
chiefly  of  paper  and  rags,  amounted,  in  1910,  to  208  Ibs.  or 
28.6  Ibs.  of  dry  material,  per  million  gallons  of  sewage.  The  wet 
screenings  are  placed  in  perforated  cans  about  18  in.  in  diam.  X  20 
in.  in  height,  which  weigh  30  Ibs.  each  and  hold  about  230  Ibs. 
of  screenings,  in  which  the  material  is  removed. 

Waterbury,  Conn. — With  a  1/2-in.  mesh  of  galvanized  wire, 
140  Ibs.  or  0.08  cu.  yd.  of  screenings  were  removed  per  million 
gallons  of  sewage.1  The  average  composition  of  the  screenings 
for  the  year  ending  November,  1906,  was  as  follows: 

TABLE  VI 
COMPOSITION  OF  SCREENINGS 


Parts  per  million 

Total 

Dissolved 

Suspended 

Oxygen  consumed  
Organic  nitrogen 

46 
14.8 
Total 
165 
26 
84 
15.7 

26 
10.3 
Fixed 
115 

20 
4.5 
Volatile 
50 

Suspended  matter  
Fats  

Particles  in  micro-suspension  
Colloidal  matter 

1  W.  G.  Taylor,  Eng.  Rec.,  June  3,  1909. 
14 


210 


SEWAGE  SLUDGE 


Plainfield,  N.  J. — The. volume  of  sewage  is  about  90  gallons 
per  capita.1  With  screens  having  openings  about  1/2  in.  apart 
in  the  clear,  0.18  to  0.22  cu.  yds.  of  screenings  containing  85 
per  cent,  of  moisture  are  removed  per  million  gallons  of  sewage. 

Pawtucket,  R.  I. — The  strong  domestic  sewage,  amounting  to 
an  average  of  0.277  million  gallons  per  day  in  1910,  passes  through 
one  of  a  pair  of  rack  screens  7.96  ft.  wide  X  4.4  ft.  high  composed 
of  wooden  strips  3/4  in. X3  in.  in  size,  spaced  5/8  in.  apart. 
The  depth  of  sewage  passing  the  screen  is  about  2.1  ft.  During 
1910,  1036  cu.  yds.  of  wet  material  were  removed  and  buried  in 
pits,  amounting  to  10.25  cu.  yds.  per  million  gallons.  This 
large  amount  is  accounted  for  in  part  by  the  fact  that  it  includes 
a  small  amount  of  grit,  which  is  pumpted  out  once  a  week  from 
the  depressed  pit  in  front  of  the  screen,  together  with  the  screen- 
ings, by  an  Edson  diaphragm  pump.  Another  reason  lies  in 
the  strength  of  the  sewage  and  to  the  fact  that  it  enters  the  screen 
chamber  3  ft.  below  the  surface  of  the  liquid.  The  screenings 
consist  of  rags,  paper,  grease,  fecal  matter^and  kitchen  wastes. 
During  the  interval  between  cleaning  a  mat  of  grease  and  other 
wastes  frequently  forms  in  the  screen  chamber,  sometimes  to 
the  thickness  of  18  in.  or  2  ft.,  of  sufficient  strength  to  support  a 
man,  below  which  the  material  is  much  more  dilute.2 

TABLE  VII 
MATERIAL  REMOVED  BY  SCREENS 


Cu.  yds.  wet 
screenings 

Lbs.   dry 
solids  per 

Per  cent,  of  suspended  solids 
in  wet  sludge.     (By  volume) 

per  million 
gal.  sewage. 

million  gal. 
sewage 

Total 

Volatile 

Fixed 

Nov.  10,  1905-Feb.  23,  1906  .  . 

7.53 

746 

5.68 

4.45 

1.23 

Mar.  2,  1906-May  11,  1906.  .  . 

5.83 

549 

5.44 

4.36 

1.08 

Oct.  12,  1906-May  3,  1907  .  .  . 

8.96 

792 

5.22 

4.15 

1.07 

Feb.  3,  1911-Mar.  24,  1911  ... 

10.25 

1024 

5.89 

5.45 

0.44 

| 

IV.  SLUDGE  FROM  PLAIN  SEDIMENTATION 

Massachusetts    State    Board    of   Health.3 — Experiments    were 
made  with  Lawrence  (combined  system)  sewage  during  the  years 

*Eng.  News.,  Vol.  LXIII,  p.  541. 

2  George  A.  Carpenter,  city  engineer 

3  Rep.  1908,  p.  454,  et  seq. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    211 


1892  to  1897,  inclusive,  by  allowing  it  to  settle  while  quiescent 
for  4  hours.  Nearly  60  per  cent,  of  the  suspended  matter  and 
33  per  cent,  of  the  total  organic  matter,  as  indicated  by  the  al- 
buminoid ammonia,  were  removed. 

In  1906  a  large  tank  was  used  for  this  purpose.  The  period 
of  sedimentation  varied  from  2  to  14  hours,  and  in  2  1/2  years  of 
operation  there  were  removed  about  44  per  cent,  of  the  suspended 
matter,  as  indicated  by  the  albuminoid  ammonia  in  suspension, 
"58  per  cent,  as  shown  by  total  solids,  and  52  per  cent,  as 
shown  by  loss  on  ignition."1 

Experiments  have  also  been  conducted  since  1903  with  the 
sewage  of  Andover,  which  passed  at  an  average  rate  of  150,000 
gallons  per  day  through  a  tank  holding  13,500  gallons,  the 
average  period  of  sedimentation  being  about  2  hours. 

"The  average  removal  of  suspended  matter  by  this  tank  was 
about  56  per  cent,  as  shown  by  determinations  of  albuminoid 
ammonia  in  suspension,  71  per  cent,  as  shown  by  total  solids, 
and  70  per  cent,  as  shown  by  loss  on  ignition."  About  31  per 
cent,  of  the  total  organic  matter  was  removed. 

TABLE  VIII 
AVERAGE  SOLIDS  IN  EFFLUENTS  FROM   TANKS.     PARTS  PER  MILLION 


Total 

Loss  on  ignition 

Fixed 

Experiment  Sta.,  1906-1908: 
Unfiltered  

624 

213 

411 

Filtered 

549 

156 

393 

In  suspension  

75 

57 

18 

Andover.,  1905-1908: 
Unfiltered  .  .  .  

446 

206 

240 

Filtered 

388 

158 

230 

In  suspension  

58 

48 

10 

Between  July  and  November  15,  1905,  measurements  of  the 
sludge  were  made,  giving  1.25  tons  per  million  gallons  of  sewage. 
Between  April  23  and  November  15,  1906,  when  the  daily  flow 
varied  from  75,000  to  350,000  gallons,  about  2.28  tons  of  wet 
sludge  were  removed  per  million  gallons  of  sewage,  assuming 

1  Rep.  1908,  p.  454,  et  seq. 


212  SEWAGE  SLUDGE 

the  average  flow  to  have  been  175,000  gallons  per  day.  This 
sludge  lost  61  per  cent,  in  weight  by  drying.  Analyses  of  the 
dried  sludge  resulted  as  follows: 

Organic  matter 60      per  cent. 

Carbon 33      per  cent. 

Organic  nitrogen 1.6  per  cent. 

Fat 24      per  cent. 

Worcester  Experiments. — In  1903  a  tank  having  a  capacity 
of  344,000  gallons  received  an  average  of  one  million  gallons  of 
sewage  per  day,  which  was  therefore  subjected  to  8  hours7 
sedimentation. 

The  suspended  matter  removed,  as  indicated  by  the  albumin- 
oid ammonia,  averaged  40.80  per  cent.,  and  the  total  organic 
matter  removed  averaged  27.48  per  cent.  The  resulting  volume 
of  sludge  was  0.125  per  cent,  of  the  sewage,  or  about  6.17  cu.  yds. 
per  million  gallons,  and  the  water  contained  was  96.5  per  cent. 
The  cost  of  pressing  this  sludge  into  cakes  was  "$1.56  per  million 
gallons  of  raw  sewage,  including  handling  of  pressed  cake  by 
trolley  to  the  sludge  dump.  The  other  costs  of  sedimentation, 
labor  and  attendance,  are  given  as  $1.85  per  million  gallons/'1 

The  composition  of  this  sewage  was,  in  parts  per  million: 

Free  ammonia 17 . 69 

Albuminoid  ammonia, 

Total 8.32 

Dissolved 2.88 

Suspended 5 . 44 

Oxygen  consumed, 

Unfiltered 93.90 

Filtered 53 .60 

Chlorine 90.40 

Columbus  Experiments.2 — The  two  tanks  used  here  were  8  ft. 
deep  and  40  ft.  long,  and  their  effective  capacity  was  about 
17,000  gallons.  The  time  of  retention  in  these  tanks  was  8 
hours  in  Tank  A  and  6  hours  in  Tank  B,  giving  respective 
velocities  of  4.9  and  6.7  ft.  per  hour  (.42  and  .56  mm.  per  second). 
The  results  were  as  follows  when  using  raw  sewage  that  had  first 
passed  through  grit  chambers: 

1  Geo.  W.  Fuller,  Trans.  Am.  Soc.  C.  E.,  Vol.  LIV,  Part  E,  p.  178. 

2  Rep.  Sewage  Purification,  1905.      Johnson,  pp.  88-91. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    213 

TABLE  IX 
RESULTS  OF  PLAIN  SEDIMENTATION  WITH  RAW  SEWAGE 


Tank  A 

Tank  B 

Influent 

Effluent 

Influent          Effluent 

Parts  per  million 

Period  of  operation 

Aug.,  1904  to  June,  1905. 

Nov.,  1904  to  April,  1905 

Oxygen  consumed  
Organic  nitrogen         

46 
8.0 
11.7 

950 
803 
147 

168 
104 
64 

782 
699 
83 

37 
6.4 
11.7 

875 
797 
78 

137 
103 
34 

738 
694 
44 

47 
7.6 
.   11.3 

927 
793 
134 

164 
100 
64 

763 
693 
70 

39 
6.3 
11.0 

857 
784 
73 

137 
99 
38 

726 
685 
35 

Free  ammonia              

Residue  on  evaporation: 
Total 

Volatile: 
Total 

Suspended 

Fixed: 
Total                            

Dissolved                      

Suspended         .         

Percentages  of  removal 

Oxygen  consumed  
Organic  nitrogen  

20 
10 
0 

47 
47 
53 

17 
17 
6 

46 
41 
50 

Residue  on  evaporation: 
Total 

Volatile 

With  raw  screened  sewage  containing  about  210  parts  per 
million  of  suspended  matter,  or  7  1/2  cu.  yds.  of  sludge  87  per 
cent,  water,  the  following  results  were  obtained.1 


Rep.  on  Sew.  Purif.,  Columbus,  1905.     Johnson,  pp.  151-153. 


214 


SEWAGE  SLUDGE 


12345678 

Capacity    in    Hours    Flow. 

FIG.  39. — Results  of  sedimentation,  Columbus.      Reproduced  by  permission  of  the 
Metropolitan  Sewage  Commission  of  N.  Y. 


TABLE  X 
RESULTS  OF  PLAIN  SEDIMENTATION  WITH  RAW  SCREENED  SEWAGE 


Tank  A 


Tank  B 


Period  of  sedimentation 

Suspended  matter  removed: 

Total 

Volatile '. 

Total  organic  matter  removed: 

Nitrogenous 

Carbonaceous 

Fats  removed.  . 


Average  period  of  sedimentation 

Wet  sludge  per  million  gallons  of  sewage 


8  hours. 

66  % 

58% 

31  % 
31  % 
50% 


6  hours. 

63  % 

54% 

30% 

26% 


6 . 8  hours. 
5.75  cu.  yds. 


Philadelphia  Experiments. — Two  tanks  were  first  used.     Tank 
No.  12  had  a  ratio  of  length  to  depth  of  1.5:1  and  a  capacity  of 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    215 


9943  gallons,  which  was  later  reduced  by  sloping  the  bottom 
and  adding  a  baffle  and  scum  boards  to  8738  gallons.  Tank 
No.  13  had  a  ratio  of  length  to  depth  of  2.5:1  and  a  capacity  of 
7767  gallons,  later  reduced  as  in  the  case  of  Tank  No.  12,  to 
5475  gallons. 

TABLE  XI 
RESULTS  OF  PLAIN  SEDIMENTATION.     TANKS  12  AND  13 


Conditions 

Hours  storage 

Suspended  solids 

Parts  per  million 

Per  cent, 
removal 

Average  effluent 

No.  12 

No.  13 

No.  12 

No.  13 

No.  12 

No.  13 

Flat  unbaffled  

6 

4  1/2 
3  1/2 

6 

4  1/2 
3  1/2 
5.85 

55 
53 
71 
60 

66 
60 
81 
75 

65.5 

72.2 
64.1 
67 

59.2 
69.3 
61 
58.7 

Flat  unbaffled 

Sloping  bottom  baffle  and  scum 

Sloping  bottom  baffle  and  scum 

Tank  No.  17  had  a  ratio  of  length  to  depth  of  4. 

TABLE  XII 

RESULTS  OF  PLAIN  SEDIMENTATION.     TANK  17 


Suspended  solids 

Conditions 

Hours 

storage 

Parts  per  million 

Per  cent. 

* 

Average  effluent 

removal 

Unbaffled 

10 

65 

50  4 

Unbaffled  (stronger  sewage) 

6 

About  65 

67  5 

Unbaffled    . 

4 

44 

81  2 

Baffles  and  scums  

4 

59 

72.5 

Baffles  and  scums  

10 

46 

74 

The  conclusion  drawn  from  these  experiments  was  that  "long 
storage  periods  are  unnecessary  for  efficient  sedimentation  and 
that  great  improvement  in  the  uniformity  of  the  tank  liquor  is 
obtained  by  efficient  baffling,  creating  uniform  velocity  over  the 
entire  area  of  the  cross  section." 

A  heavy  scum  formed  on  Tank  No.  13,  which  received  un- 
screened sewage.  After  introducing  baffles  this  formed  to  a 
thickness  of  2  ft.,  amounting  to  5.6  cu.  yds.,  weighing  5  tons. 


216 


SEWAGE  SLUDGE 


It  contained  1810  Ibs.  of  dry  solids  and  260  Ibs.  of  fat,  and  was 
very  offensive  when  punctured  or  removed. 

The  average  composition  of  the  sludge  was  as  follows: 


TABLE  XIII 
COMPOSITION  OF  SLUDGE 


Tank  No 

12 

13 

17 

Sewage  

Screened 

Crude 

Crude 

Wet  sludge: 
Specific  gravity 

1  036 

1  053 

1  043 

Per  cent,  moisture  

90 

86  1 

87  7 

Per  cent,  of  dry  residue: 
Volatile    .  . 

49 

48 

50 

Fixed  

51 

52 

50 

Nitrogen  
Fat  

1.3 

8.1 

1.4 

7.4 

1.3 

7.2 

The  sludge  from  Tank  No.  12,  which  had  been  fine-screened, 
was  uniform  and  with  no  particle  over  1  mm.  diameter.  It 
therefore  flowed  much  more  freely  than  that  from  Nos.  13  and  17, 
which  contained  fibers  of  wool  and  hops. 

Scum  formed  in  irregular  amounts  in  these  tanks  before  placing 
scum  boards,  but  after  this  was  done  it  formed  promptly  and 
increased  to  a  considerable  thickness  at  the  inlet  end  of  the  tank, 
being  tough  and  tenacious. 

The  following  is  a  typical  analysis  of  this  material  when 
formed  on  crude  sewage: 

.Average  characteristics  of  scum  from  Tanks  No.  13,  No.  17 
and  No.  19  (Emscher). 

Specific  gravity 1 . 05  per  cent. 

Moisture 82 . 5    per  cent, 

Dry  residue 17.5    per  cent. 

The  average  composition  of  the  dry  residue  was: 

Volatile  matter 60 . 5  per  cent. 

Fixed 39 . 5  per  cent. 

Nitrogen 1.7  per  cent. 

Fats 13.5  per  cent. 

No  scum  of  this  kind  formed  on  Tank  No.  12. 

The  fact  that  the  scum  floats  in  spite  of  its  high  specific  gravity 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    217 


is  explained  by  the  presence  of  entrained  bubbles  of  gas  which, 
when  liberated  on  removal,  produced  an  offensive  odor. 

Reading,  Pa. — Sedimentation  here  for  15  hours  removed  123 
of  the  165  parts  per  million  of  suspended  matter.1  During  1910 
a  sedimentation  tank  at  Millmont  received  1425  million  gallons 
of  sewage. 

Three  and  three-tenths  cubic  yards  per  million  gallons  were 
removed  with  10  hours'  retention  and  a  flow  of  3  million  gallons 
per  day  through  a  tank  250  ft.X50  ft.Xl6  ft.  in  size.  The 
composition  of  the  sludge  was:2 

Moisture 91 . 83  per  cent. 

Mineral  matter 2 .83  per  cent. 

Volatile  matter 5 . 34  per  cent. 

The  tank  was  cleaned  6  times  during  the  year,  but  at  no  time 
have  there  been  seriously  objectionable  odors  from  the  sludge. 

Kinnicutt,  Winslow  and  Pratt  give  the  following  comparison 
of  the  results  obtained  by  plain  sedimentation  at  Plain'field,  N.  J. ; 
Columbus,  O.,  and  Reading,  Pa. 

RESULTS  OF  PLAIN  SEDIMENTATION 


Period  of  sedimentation 

Suspended  solids 

Per  cent. 

Hours 

Parts  per  million 

Influent 

Effluent 

Reduction 

Plainfield,  N.  J..T  
Columbus,  O  
Reading,  Pa  

10.0 
13.0 
15.0 

118 
304 
165 

54 
101 
42 

54 
67 
75 

V.  SEPTIC  TANK  SLUDGE 

Massachusetts  State  Board  of  Health.3 — Experiments  with 
septic  tanks  have  been  conducted  at  Lawrence  since  1898  and 
with  Andover  sewage  from  July,  1899,  to  July,  1902.  In  five 
series4  of  experiments  the  analyses  of  the  sewage  and  effluent 
varied  between  the  following  limits: 

1  Kinnicutt,  Winslow  and  Pratt. 

2  Eng.  Rec.,  Vol.  LXII,  p.  186. 

3  Rep.  1908,  p.  476,  et  seq. 

A  sixth  series  in  which  sludge  was  used  in  place  of  sewage  is  omitted. 


218 


SEWAGE  SLUDGE 


TABLE  XIV 
SUSPENDED  MATTER  IN  SEWAGE  AND  EFFLUENT 


Parts  pe 

r  million 

Entering  sewage 

Tank  effluent 

Unfiltered:  Total  
Loss  on  ignition  
Filtered:  Total 

646-912 
323-464 
475-537 

493-571 
175-232 
448-510 

Loss  on  ignition 

174-199 

126-173 

In  Tank  A,  70  per  cent,  of  the  total  suspended  matter  and  70 
per  cent,  of  the  suspended  organic  matter  received  during  61/4 
years  were  deposited,  and  of  this  82  per  cent,  of  the  total  and  88 
per  cent,  of  the  organic  suspended  matter  were  destroyed. 
Again,  during  a  period  of  4  1J2  years,  66  per  cent,  of  the  total 
suspended  matter  and  66  per  cent,  of  the  suspended  organic 
matter  were  deposited,  and  of  this  about  two-thirds  of  the  total 
and  80  per  cent,  of  the  suspended  organic  matter  were  destroyed 
by  digestion;  The  period  of  sedimentation  averaged  in  the  first 
case,  about  12  hours,  and  in  the  second  case,  15  hours. 

In  Tank  G,  in  which  the  period  of  sedimentation  was  about  6 
hours,  60  per  cent,  of  both  the  total  and  organic  suspended 
matter  was  deposited. 

In  Tank  H,  with  18  hours'  storage,  75  per  cent,  of  the  total  and 
78  per  cent,  of  the  suspended  organic  matter  were  deposited, 
while  84  per  cent,  of  the  total  and  90  per  cent,  of  the  organic 
matter  which  deposited  were  destroyed. 

In  Tank  F,  which  received  a  sewage  with  about  50  per  cent, 
more  suspended  matter  than  Tank  A,  and  more  than  twice  that 
of  Tanks  G  and  H,  76  per  cent,  of  the  total  and  82  per  cent,  of  the 
organic  suspended  matter  were  deposited,  while  of  this,  71  per 
cent,  of  the  total  and  86  per  cent,  of  the  organic  matter  were 
destroyed. 

Tank  B  received  a  sewage  about  10  times  as  strong  as  G  and  H. 
Here  82  per  cent,  of  the  total  and  84  per  cent,  of  the  organic 
suspended  matter  were  deposited,  and  74  per  cent,  of  the  total 
and  82  per  cent,  of  the  organic  suspended  matter  in  this  were 
destroyed. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    219 


TABLE  XV 

C«».M POSITION  OF  DRY  SEPTIC  TANK  Si.i  IH.I; 


Per  cent. 

Number  of  tanks  from 
which  sludge  was  sampled 

Mineral  matter 

45  6  to  70  9 

6 

Total  organic  matter  
Organic  nitrogen  .  .  

54  .  4  to  29  .  1 
1.1  to    2.9 

6 
6 

Fats  !  
Carbon  
Hydrogen  

8.8  to  11.9 
25.1  to  29.  8 
3  .  0  to    4.0 

4 
3 
3 

Madison,  Wis. — Here,  by  septic  tank  treatment,  42.8  per  cent, 
of  the  suspended  solids  and  60.8  per  cent,  of  the  albuminoid 
ammonia  are  removed.1 

TABLE  XVI 

SUMMARY  OF  RESULTS  SHOWING  ACCUMULATION  AND  LIQUEFACTION  OF 

SLUDGE 


Septic  tank 

A 

B 

C 

D 

E 

Aug. 

Aug. 

Nov. 

Feb. 

Mar. 

• 

16 

16 

22 

18 

9 

Period  of  service  1904-5 

to 

to 

to 

to 

to 

June 

June 

June 

June 

June 

30 

30 

30 

30 

30 

Days  in  service  

300 

301 

221 

132 

118 

Total  million  gallons  of  sewage  treated 

9.5 

5.7 

10.8 

4.  1 

0.79 

Average  period  of  flow  (hours) 

13.9 

21  .8 

8.0 

4.0 

8.0 

Average  velocity  of  flow  millimeters  per  second  

0.24 

0.15 

0.42 

0.84 

0.14 

Tons  dry  solids  per  million  gallons: 

In  applied  sewage  

0.61 

0.61 

0.62 

0.86 

1.21 

Deposited  in  tank  

0.31 

0.31 

0.28 

0.26 

0.67 

Escaped  in  effluent  

0.30 

0.30 

0.34 

0.66 

0.54 

Tons  dry  solids  per  million  gallons: 

Deposited2 

0.40 

0.40  * 

0.36 

0.33 

d.86 

In  tank  at  end  of  tests   

0.21 

0.29 

0.12 

0.20 

0.43 

Per  cent,  solid  matter  liquefied  

48 

28 

67 

39    . 

50 

Cubic  yards  wet  sludge  per  million  gallons  sludge 

1.4 

1       K 

0.8 

1.5 

2.9 

treated  found  in  tank  at  end  of  test. 

Columbus  Experiments. — Five  septic  tanks  were  used  in  the* 
Columbus  experiments.  Tanks  A,  B,  C  and  D  received  the 
effluent  from  the  grit  chamber,  and  A  received  the  sewage  after 
screening.  Tanks  A,  B  and  C  were  8  ft.  X40  ft.  in  plan  with 

1  Purification  with  special  ref.  to  Wis.  conditions.      Geo.  J.  Davis,  Jr.,  and  J.  T.  Bowles. 
Bui.  Univ.  Wis.,  Oct.,  1909. 

2  Corrected  in  ratio  of  28  to  36  to  correspond  to  ratio  of  computed  deposit  to  actual 
deposit  respectively  in  plain  sedimentation  Tank  A. 


220 


SEWAGE  SLUDGE 


an  effective  capacity  of  17,000  gallons.  Tank  D  was  circular, 
12  1/2  ft.  in  diameter,  with  an  effective  depth  of  5  1/2  ft.  It 
was  baffled  so  as  to  make  the  length  of  flow  40  ft.  and  had  an 
effective  capacity  of  about  5370  gallons,  and  was  covered. 
Tank  E  was  a  cylindrical  boiler  shell  6  ft.  diameter  by  15  ft.  long, 
air-tight,  with  a  1/2-in.  gas  pipe  leading  to  a  rneter.  The  effec- 
tive depth  was  5  ft.  and  the  capacity  7200  gallons. 

In  general,  it  may  be  assumed  that  in  Tanks  A,  B  and  C  the 
average  accumulated  deposit  amounted  to  1.33  cu.  yds.  per 
million  gallons  as  compared  with  3.3  cu.  yds.  per  million  gallons 
with  plain  sedimentation.  The  percentage  liquefied  was  there- 
fore about  60.  The  50  per  cent,  liquefied  with  crude  sewage 
in  Tank  E  was  believed  to  be  a  fair  average  to  use  in  estimates. 

The  composition  of  the  resulting  septic  sludge  was  found  by 
analysis  to  be  as  follows: 

TABLE  XVII 
COMPOSITION  OF  SEPTIC  TANK  SLUDGE 


Tank 

A 

B 

C 

D 

E 

Weight  of  wet  sludge   pounds  per  cubic  yards  

1836 

1823 

1823 

1800 

1833 

1  089 

1.080 

1.080 

1.069 

1.087 

83.3 

82.3 

83.2 

84.7 

83.7 

Solids,  per  cent  

16.7 
4.4 

17.7 
4.4 

16.8 
4.3 

15.3 
4.1 

16.3 
5.0 

0.25 

0.25 

0.23 

0.19 

0.18 

Fats                                                              

0.94 

1.06 

1.05 

1.36 

1.17 

The  reduction  of  suspended  matter  secured  at  Columbus  during 
the  years  1909  and  1910  by  septic  tank-treatment  was  as  follows: 

TABLE  XVIII 
REDUCTION  OF  SUSPENDED  MATTER* 


Maximum 

Minimum 

Mean 

1909 

1910 

1909 

1910 

1909 

1910 

Daily  volume  of  sewage,  million  gallons  21  .4 
Period  of  flow  through  tank,  hours  36  0 

17.9        3.1 
17.0        4.3 

2.5 

3.5 

11.1 
10.1 

12.9 

7.2 

Total  suspended  matter,  parts  per  million  : 
Screened  sewage  
Septic  effluent  

1088 
230 

630 
264 

13 

12 

16 
9 

201 

82 

211 
80 

i  Furnished  by  W.  W.  Jackson,  Supt.  Water  Works. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    221 

Pawtucket,  R.  I.1 — In  1900,  41.5  per  cent,  of  the  organic  matter 
was  removed  by  the  tanks.  At  the  end  of  10  months'  operation 
the  accumulation  of  sludge  was  5.428  cu.  yds.  per  million  gallons 
with  81.75  per  cent,  moisture.  The  dried  solids  amounted, 
therefore,  to  0.99  cu.  yds.  per  million  gallons  of  sewage.  377.33 
parts  per  million,  or  1.868  cu.  yds.  of  mineral  matter  were  de- 
posited in  the  tank  for  each  million  gallons  of  sewage.  The 
amount  of  dried  solids  removed  by  septic  action  was,  therefore, 
1.87-0.99  =  0.88  cu.  yds.  per  million  gallons.  The  cost  of  its 
removal  was  411/3  cts.  per  cubic  yard.  This  treatment  has  now 
been  discontinued. 

Mansfield,  O.2 — The  sewage  from  about  10,000  persons,  40  per 
cent,  of  which  is  collected  by  the  separate  system  and  which  is 
much  diluted  with  ground  water,  flows  at  a  rate  of  about  one 
million  gallons  per  day  through  bar  screens  with  3/4-in.  openings, 
and  is  then  settled  in  4  septic  tanks  92  ft.  3  in.  X52  ft.  in  plan, 
having  an  effective  depth  of  7  ft.  and  a  combined  capacity  of  1 
million  gallons. 

The  crude  influent  sewage  contains  but  34  to  42  parts  per 
million  of  suspended  matter,  and  the  effluent  contains  34  parts 
per  million,  showing  very  little  reduction.  The  following  is  a 
more  recent  analysis.  Parts  per  million  of  suspended  matter: 

Total         Volatile 

Crude  sewage 74  55 

Septic  effluent 85  to  135     43  to  54 

The  sludge  resulting  from  4  years'  operation  weighed  1868 
Ibs.  per  cubic  yard.  It  had  a  specific  gravity  of  1.11,  and  con- 
tained: 

Moisture 80 . 8  per  cent.     Nitrogen 1 . 03  per  cent. 

Volatile  matter.  3 . 6  per  cent.     Fats 4.7    per  cent. 

It  was  granular  in  structure  and  not  offensive  when  removed 
from  the  tanks.  Exposed  in  thin  layers  for  about  4  days  the 
black  color,  due  to  ferric  sulphide,  disappeared,  leaving  the 
material  similar  to  humus. 

The  cost  of  disposal  for  the  1200  cu.  yds.  of  sludge  removed 
was  about  50  cts.  per  cu.  yd. 

Plain fidd,  N.  J.3 — The  population  of  20,550  persons  furnishes 

1  Rep.  of  City  Eng'r  for  year  ending  Sept.  30,  1900. 

2  Rep.  St.  Bd.  Hlth.,  1908. 

3  Eng.  Rec.  Vol.  LXIV,  p.  29. 


222 


SEWAGE  SLUDGE 


about  1.9  million  gallons  per.  day  of  domestic  sewage  to  4  septic 
tanks  having  a  combined  capacity  of  1.35  million  gallons. 

In  March,  1910,  1600  cu.  yds.  of  wet  sludge  and  scum  were 
removed,  equivalent  to  3.35  cu.  yds.  per  million  gallons  of  sewage 
treated  during  the  previous  11  months. 

In  March,  1911,  1650  cu.  yds.  of  wet  sludge  and  scum  were 
removed,  equivalent  to  3.01  cu.  yds.  per  million  gallons  of  sewage 
treated  during  the  previous  year. 

No  objectionable  odors  were  given  off  except  while  the  tanks 
were  being  emptied. 

In  1910  the  average  suspended  matter,  in  parts  per  million, 
was  as  follows: 

In  screened  sewage  152,  varying  from  114  in  February  to  271 
in  November. 

In  septic  effluent  56,  varying  from  42  in  May  to  72  in  December. 

The  percentage  of  removal  was,  therefore,  64.5. 

The  fats  averaged  42.8  parts  per  million  in  the  screened  sewage, 
and  27.7  parts  per  million  in  the  septic  effluent,  the  percentage 
of  removal  being  about  35. 

Waterbury,  Conn.1 — Observations  were  made  here  of  the 
results  obtained  with  two  septic  tanks  14  ft.  X6  ft.  3  in.  X6  ft. 
in  size,  of  a  capacity  of  nearly  4000  gallons  each,  beginning  in 
June,  1905,  and  lasting  18  months.  The  time  allowed  for 
sedimentation  varied  from  8  to  33  hours,  and  the  results  were  as 
follows: 

TABLE  XIX 
REMOVAL  OF  SOLIDS  IN  SEPTIC  TANKS 


Tank  No.  2 

Tank  No.  3 

Average  period  of  sedimentation.     Hours.  .  .  . 
Horizontal  vel   in  mm   per  sec 

15.5 
0  08 

11. 
0.11 

Wet  sludge  in  tank.     Cu.  yds.  per  million  gal- 
lon sewage. 
Dry  solids  deposited.     Tons  per  million  gal- 
lon sewage. 
Per  cent   retained  in  tank                              .    ... 

1.07 
0.25 
56 

0.55 
0.25 
36 

Eng.  News,  Vol.  LXI,  p.  596. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    223 


TABLE  XX 

COMPOSITION  OF  SEPTIC  SLUDGE  AND  SCUM  IN  TERMS  OF  THE  WET  MATERIAL 


Sludge 

Scum 

Tank  No.  2 

Tank  No.  3 

Tank  No.  2 

Pounds  per  cubic  yard 

1721 
1.02 
86.3 
13.7 
5.9 
0.16 
1.53 

1738 
1.03 
85.4 
14.6 
7. 
0.22 
1.52 

1637 
0.97 
80.9 
19.1 
8.9 
0.34 
2.00 

Specific  gravity  

Moisture,  per  cent  
Total  solids,  per  cent 

Volatile,  per  cent  
Nitrogen,  per  -cent  
Fats,  per  cent  

Saratoga,  N.  Y.1 — The  volume  of  flow  amounted  in  1904,  to 
from  1  1/4  to  2  1/2  million  gallons  daily  of  weak  domestic 
sewage,  the  population  varying  from  12,000  in  winter  to  50,000  in 
summer. 


COMPOSITION  OF  SEWAGE 


Free  ammonia 

Albuminoid  ammonia 
Oxygen  consumed  .  .  . 
Suspended  solids 


Parts-  per  million 

20 

4 

50       • 

.    200 


There  are  4  septic  tanks  91  1/2x51. 1/2  ft.,  holding  8  ft.  depth 
of  sewage,  or  with  a  total  capacity  of  1,000,000  gals. 

The  period  of  retention  of  sewage  was  10  to  15  hours.  From 
July,  1903,  until  January,  1905,  no  sludge  was  removed  from 
septic  tanks. 

The  following  shows  the  results  of  the  treatment: 

Total  dry  solids  received 500  tons. 

Dry  solids  passed  in  effluent 175  tons  =  35  per  cent.. 

Dry  solids  in  tank,  Jan.  1,  1905 100  tons  =  20  per  cent. 

Dry  solids  removed  by  digestion 225  tons  =  45  per  cent. 

1  Eng.  Rec.,  Vol.  LI,  p.  84  and  Rep.  N.  Y.  Dept.  Hlth.,  1907,  Vol.  II. 


224  SEWAGE  SLUDGE 

The  composition  of  the  sludge  and  scum  was  as  follows : 


Sludge 

Scum 

Wet  material: 

Specific  gravity  

1.025 

0.975 

Moisture  

94  .  0  per  cent. 

86  .  5  per  cent. 

Dry  residue: 

Volatile  

4  .  5  per  cent. 

10.0  per  cent. 

Fixed 

1  5  per  cent. 

3  .  5  per  cent. 

VI.  SLUDGE  FROM  EMSCHER  TANKS 

Philadelphia  Experiments. — Experiments  were  made  in  Phila- 
delphia with  an  Emscher  tank  5  ft.  in  diameter  and  10  ft.  deep. 
The  conical  bottom  inclined  30  degrees  to  the  horizontal.  By 
a  cylindrical  baffle,  the  motion  of  the  sewage  was  first  downward 
from  the  annular  influent  channel  surrounding  the  central  vent 
and  then  upward  to  the  effluent  at  the  periphery — about  41/2 
ft.  in  each  direction.  The  sludge  chamber  was  about  4  ft.  in 
effective  depth.  The  time  of  passage  was  about  2  hours.  Dur- 
ing 3  months'  use  (Jan.  12  to  April  13,  1910)  53  per  cent,  of  the 
suspended  solids  were  removed,  leaving  in  the  effluent  92  parts 
per  million. 

The  sludge  produced  had  the  following  characteristics: 

Wet  sludge:  , 

Specific  gravity 1  •  085 

Moisture 82 . 5  per  cent. 

Volume  per  million  gals,  of  sewage 0.9  cu.  yds. 

Dry  residue: 

Volatile 38      per  cent. 

Fixed 62      per  cent. 

Nitrogen 1 . 2  per  cent. 

Fats 6 . 5  per  cent. 

The  results  obtained  are  not  strictly  comparable  with  those 
from  a  tank  of  the  30  ft.  depth  recommended  by  Dr.  Imhoff. 
The  deeper  tank  would  produce  a  sludge  with  less  moisture  and 
with  a  larger  amount  of  entrained  gas,  which  would  be  of  subse- 
quent value  in  assisting  the  process  of  drying. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    225 

The  evolution  of  gas  appears  to  have  been  quite  active  in  the 
sludge  chamber.  It  was  inodorous  and  presumably  composed 
chiefly  of  methane  (CH4). 

As  withdrawn  from  the  tank  the  sludge  was  ''fine,  granular 
and  homogeneous;  considering  its  relative  dryness,  it  flowed 
freely  and  did  not  have  an  offensive  odor.  When  withdrawn 
from  the  sludge  outlet,  the  odor  was  decidedly  'tarry/  and 
after  a  few  days  the  dried  mass  was  inodorous."  The  solids 
appeared  to  have  been  completely  digested. 

The  composition  of  the  scum  was  as  follows: 

Wet  sludge: 

Specific  gravity 1 . 05 

Moisture 87 .2  per  cent. 

Dry  residue: 

Volatile 61.8  per  cent. 

Fixed 32 .8  per  cent. 

Nitrogen 1.9  per  cent. 

Fats 14.3  per  cent. 

Chicago  Experiments.1 — Experiments  with  the  Emscher  tank 
have  been  carried  on  by  the  Chicago  Sanitary  District. 

The  total  depth  of  the  circular  tank  was  17  ft.  and  the  inside 
diameter  7  ft.  6  1/2  in.  An  18-in.  central  vent  pipe  was  supported 
by  a  conical  hood  separating  the  two  chambers  and  a  cylindrical 
baffle  caused  the  sewage  entering  near  the  central  vent  to  descend 
to  a  depth  of  at  least  3  ft.  from  the  surface  and  then  rise  to  the 
opening  leading  to  the  2-in.  effluent  pipe.  The  sludge  chamber 
had  a  total  depth  of  12  ft.  3  in.,  the  lower  4  ft.  forming  a  cone 
with  a  slope  of  45  degrees. 

The  sewage  flow  amounted  to  48,500  gallons  per  day  from  May 
26  to  June  7, 1910,  31,000  gallons  per  day  from  June  8  to  Sept.  12, 
and  then  13,500  gallons  per  day  to  Nov.  1.  It  was  first  passed 
through  a  5/8-in.  screen  and  then  pumped  from  midrdepth  at 
the  screen  chamber  through  400ft.  of  force  main  and  a  grit  cham- 
ber to  the  tank.  The  time  of  passage  through  the  tank  was 
about  2  hours,  the  latter  part  of  the  time  the  capacity  of  the 
upper  chamber  being  1175  gallons.  The  reduction  of  suspended 
matter  was  from  64  per  cent,  to  69  per  cent,  after  the  operation 
was  well  established.  About  2  cu.  yds.  of  sludge  were  produced 

1  George  M.  Wisner,  Chief  Engineer,  and  Langdon  Pearse,  Assistant  Engineer,  the  Sanitary 
District  of  Chicago. 
15 


226 


SEWAGE  SLUDGE 


per  million  gal.  of  sewage  during  the  summer  and  fall  of  1910, 
having  the  following  characteristics,  and  0.93  cu.  yds.  per  million 
gallons  of  completely  digested  sludge  per  million  gallons  during 
a  period  of  5  months.1 
Wet  sludge: 

Specific  gravity 1 . 04 

Moisture 86  to  90  per  cent. 

Dry  residue: 

Volatile  matter 39  per  cent. 

Fixed  matter 61  per  cent. 

With  prolonged  operation,  and  where  the  sludge  chambers 
are  of  sufficient  depth,  any  mixing  of  the  top  and  bottom  layers 
of  sludge  will  be  prevented  and  there  will  be  a  gradual  move- 
ment downward  toward  the  bottom.  As  the  sludge  approaches 
the  outlet,  the  organic  ingredients  are  more  and  more  decomposed 
so  that  a  more  favorable  condition,  when  discharged,  may  be 
expected  than  in  the  case  of  the  comparatively  small  experi- 
mental plants  at  Philadelphia  and  Chicago. 

The  following  table  gives  comparative  data  for  several  plants 
in  the  Emscher  district.2 

TABLE  XXI 

RESULTS  OF  TREATMENT  IN  THE  EMSCHER  DISTRICT 


J-l.CUli.lllJ. 

tuouoou 

±j\j\j 

UUJJJ. 

Sewage 

Effluent 

Sewage 

Effluent 

Sewage 

Effluent 

Suspended  solids: 

Total  

466.6 

127.5 

402.0 

93.3 

449.8 

135.4 

Organic  

259.3 

73.5 

186.9 

56.4 

261.6 

85.6 

Mineral 

207.3 

54.0 

215.1 

36.9 

188.2 

49.8 

Average    percent,    removal    of 

72 

7 

7 

7 

0 

total  suspended  solids. 

Period  of  sedimentation,  hours  . 

3/4- 

1  1/4 

1-1 

1/2 

1/2 

-1 

Sludge,  cubic  yards  per  million 

1 

65 

2 

.1 

1. 

39 

gallons. 

Per  cent,  moisture  in  sludge  .... 

82 

.93 

78 

-1 

75 

.6 

In  comparing  these  results  with  those  obtained  at  Philadelphia, 
the  German  plants  are  seen  to  handle  a  much  denser  sewage 
combined  with  a  lower  percentage  of  moisture  in  the  sludge 
resulting  from  briefer  periods  of  sedimentation.  It  should  be 
remembered,  too,  that  the  sewage  in  the  Emscher  District 

1  Rep.  on  Sew.  Disp.,  Chicago,  Geo.  M.  Wisner,  1911. 

2  Technisches  Gemeindeblatt.,  Mar.,  1911.     Eng.  News,  Vol.  LXV,  p.  663. 

3  The  sludge  space  at  Recklinghausen  is  insufficient  for  complete  decomposition. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    227 

includes  large  volumes  of  trade  ^wastes.  For  these  reasons 
further  experience  under  normal  American  conditions,  and  with 
full-sized  plants,  are  necessary  before  the  true  relative  value  of 
this  process  can  be  established. 

In  a  paper  presented  by  Mr.  Charles  Saville1  to  the  Boston 
Society  of  Civil  Engineers,2  he  states  that  from  65  to  75  per  cent, 
of  the  suspended  solids  in  the  sewage  are  usually  removed.  The 
scum  formed  is  generally  of  small  amount  and  quite  odorless. 
It  requires  loosening  with  a  rake  once  a  month  or  so,  to  permit 
the  escape  of  gas  and  allow  much  of  it  to  deposit,  but  about 
once  a  year  it  becomes  necessary  to  remove  the  scum  from  the' 
surface. 

The  sludge  is  drawn  off  at  intervals  of  from  2  weeks  to  6  months, 
preferably  the  more  frequent  period.  This  should  not  be  done 
by  suction  as  this  removes  the  entrained  gases  which  it  is  desir- 
able to  retain  incorporated  with  the  sludge  until  placed  on  the 
drying  bed;  nor  should  withdrawal  be  so  rapid  as  to  permit  the 
formation  of  a  cone  at  the  surface  and  the  consequent  entrance  of 
sewage,  as  described  by  Eisner.  The  effluent  pipe  should  then 
be  filled  with  water  or  sewage  to  prevent  the  formation  of  an 
interior  crust  and  consequent  clogging  with  the  next  dose  of 
sludge. 

The  energy  with  which  digestion  takes  place  probably  depends 
to  a  great  extent  on  the  temperature.  In  the  Emscher  District 
the  temperature  in  the  sludge  chamber  remains  practically 
between  55°  and  63°  even  in  winter.  If  the  tanks  were  above 
ground  or  in  a  much  colder  climate  the  sludge  chamber  would 
probably  have  to  be  of  greater  capacity  on  account  of  the  de- 
creased bacterial  activity. 

In  the  design  of  Emscher  tanks  the  space  required  for  sludge 
storage  should  be  approximately  known.  Until  more  is  known 
regarding  the  rate  of  progressive  concentration,  due  to  digestion, 
it  will  be  safe  to  assume  the  volume  of  the  sludge  during  storage 
a  mean  between  that  required  for  fresh  sludge  and  that  finally 
produced  by  the  Emscher  tank  from  the  sewage  treated  during 
the  assumed  period  of  storage.  The  volume  of  fresh  sludge  and 
the  period  of  retention  may  be  assumed  from  the  data  already 
given  in  the  discussions  by  Dr.  Eisner  and  Dr.  Spillner. 

If  we  assume  80  per  cent,  moisture  in  Emscher  sludge  and  90 

1  Associated  with  the  firm  of  Hering  and  Gregory.     Formerly  with  the  Emschergenossen- 
sctiaft  at  Essen. 

2  Dec.  28,  1910. 


228 


SEWAGE  SLUDGE 


per  cent,  in  freshly  settled  sludge,  the  latter  will  occupy  twice  the 
volume  of  the  former,  and  an  equal  mixture  will  occupy  11/2 
times  the  volume  of  Emscher  sludge  when  completely  digested. 
Therefore,  if: 

v  =  ftow  of  sewage  in  gallons  per  capita  per  day 

V  =  total  daily  flow  of  sewage 

p  =  population  served 

D  =  days'  retention  of  sludge 

C  =  effective  capacity  of  digestion  chamber  in  cubic  feet. 

DV 

Then,  for  combined  sewage,  C  =  10,500  —  =  10,500  PD,  and 

DV 
for  separate  sewage,  C=  5,250  —  =  5,250  PD. 

If  we  know  the  parts  per  million  of  suspended  solids  that  may 
be  expected  to  settle  out  from  a  known  sewage  in  its  passage 
through  the  sedimentation  chamber  and  if  we  accept  the  esti- 
mate of  Spillner  and  Blunk  (see  page  177),  as  to  the  reduction 
in  the  volume  of  sludge  by  its  passage  through  the  digestion 
chamber  of  the  Emscher  tank  and  by  subsequent  air-drying, 
there  will  result  the  quantities  given  in  the  following  table. 

TABLE  XXII 

VOLUME  OF  SLUDGE  AND  AIR-DRIED  SLUDGE  PER  MILLION  GALLONS  OF 
SEWAGE  OF  DIFFERENT  DENSITIES  FROM  SEPARATE  SYSTEMS  RESULT- 
ING FROM  EMSCHER  TANK  TREATMENT 


Suspended 
solids  in 
sewage 
deposited 
parts  per 
million 

Cubic  yards  fresh  sludge 

Cubic  yards 
Emscher 
sludge 
moisture 
75  per  cent.1 

Cubic  yards 
spadable 
air-dried 
sludge2 

Moisture 
95  per  cent. 

Moisture 
90  per  cent. 

25 

2.48 

1.24 

0.40 

0.16 

50 

4.95 

2.48 

0.79 

0.32 

75 

7.43 

3.72 

1.19 

0.48 

100 

9.90 

4.95 

1.58 

0.63 

125 

12.38 

6.19 

1.98                         0.79 

150                          14.85 

7.43 

2.38 

0.95 

175                          17.33 

8.17 

2.77 

1.11 

200                          19.80 

9.90 

3.17 

1.27 

225                           22  28 

11.14 

3.56 

1.42 

250                           24  .  75 

12.38 

3.96 

1.58 

275 

27  .  23 

13.12 

4.35 

1.74 

300                           29  .  70 

14.85 

4.75 

1.90 

1  Equals  16  per  cent,  of  sludge  with  95  per  cent,  moisture. 

2  Equals  40  per  cent,  of  Emscher  sludge. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    229 

Imhoff  gives  the  sludge  produced  by  the  combined  sewage  of 
Bochum  as  0.2  liters,  or  0.007  cu.  ft.  per  capita  daily.1  If  v  =  the 
sewage  flow  per  capita  daily,  the  sludge  resulting  from  each 
million  gallons  of  combined  sewage  will  be  7000/v  cu.  ft.,  or 
260/v  cu.  yds.  The  sludge  resulting  from  separate  sewage, 
Dr.  Imhoff  says,  is  about  half  as  much.  With  these  assumptions, 
the  following  table  of  sludge  volumes,  which  was  prepared  by 
Mr.  John  H.  Gregory  for  the  Metropolitan  Sewerage  Commission, 
of  New  York,  gives  the  sludge  output  that  will  require  final 
disposal. 


TABLE  XXIII 


SLUDGE  PRODUCED  BY  THE  EMSCHER  TANK  WITH  SEWAGES  OF  DIFFERENT 

STRENGTHS 


Sewage 

flow 

gallons  per 
24  hours 


Volume  of  sludge 


Separate  system 

0.0035  cu.  ft.  per  capita  per 

day  =3. 5  cu.  ft.  per  1000  pop. 

per  day. 


Combined  system 

0.007  cu.  ft.  per  capita  per 

day  =7.0  cu.  ft.  per  1000  pop. 

per  day 


Per  capita 


Cubic  feet  per 
million  gallons 


Cubic  yards  per 
million  gallons 


Cubic  feet  per 
million  gallons 


Cubic  yards  per 
million  gallons 


50          70 

2.6- 

140 

5.2- 

60 

.   58  + 

2.2- 

117- 

4.3- 

70 

50 

1.9- 

100 

3.7 

75 

47- 

1.7  + 

93  + 

3.5- 

80 

44  

1.6  + 

88- 

3.2  + 

90 

39- 

1.4  + 

78- 

2.9- 

100 

35 

1.3- 

70 

2.6 

110 

32- 

1.2- 

64- 

2.4- 

120 

29  + 

1.1- 

58  + 

2.2- 

125 

28  + 

1.0  + 

56 

2.1- 

130 

27- 

1.0- 

54- 

2.0 

140 

25 

0.93 

50 

1.9- 

150 

23  + 

0.86 

47- 

1.7  + 

160 

22- 

0.81 

44- 

1.6  + 

170          21- 

0.76 

41- 

1.5  + 

175          20 

0.74 

40 

1.5- 

180           19  + 

0.72 

39- 

1.4  + 

190          18  + 

0.68           37- 

1.4- 

200           17+           0.65           35 

1.3 

Wasser  und  Abwasser,  Feb.  4,  1911,  p.  446. 


230  SEWAGE  SLUDGE 

VII.     SLUDGE  FROM  CHEMICAL  PRECIPITATION 

Massachusetts  State  Board  of  Health.1 — A  large  number  of 
experiments  have  been  made  by  the  Massachusetts  State  Board 
of  Health  on  chemical  precipitation.  These  indicate  that  by  the 
proper  use  of  copperas,  ferric  sulphate  or  alum,  all  the  suspended 
matter  and  from  25  to  43  per  cent,  of  the  soluble  organic  matter 
of  sewage  as  indicated  by  albuminoid  ammonia  may  be  removed. 

"  Using  equal  values  of  the  different  precipitants,  applied  under 
the  most  favorable  conditions  for  each,  upon  the  same  sewage, 
the  best  results  were  obtained  with  ferric  sulphate.  Nearly  as 
good  results  were  obtained  with  copperas  and  lime,  while  lime  or 
alum  alone  gave  somewhat  inferior  effluents." 

During  the  5  years,  beginning  with  1893,  sewage  was  treated 
with  1000  Ibs.  sulphate  alumina  per  million  gallons,  and  allowed 
4  hours  for  sedimentation.  As  a  result  there  was  removed: 

Total  albuminoid  ammonia,  56  per  cent.,  varying  from  50  to 

63  per  cent.,  in  different  years. 
Albuminoid  ammonia  in  suspension,  78  per  cent.,  varying 

from  72  to  83  per  cent,  in  different  years. 
Fats,  59  per  cent.,  varying  from  47  to  80  per  cent,  in  different 

years. 

Worcester ,  Mass.2 — The  sewage  in  1910  averaged  14.57  million 
gallons  per  day  (or  107.2  gallons  per  capita),  including  3.47 
million  gallons  of  infiltration  and  a  large  amount  of  factory 
waste,  rendering  it  decidedly  acid.  Of  this  volume  an  average 
of  9.81  million  gallons  of  sewage  per  day  were  treated  with  989 
Ibs.  of  lime  per  million  gallons.  After  from  6  to  12  hours  of  pre- 
cipitation,3 the  sludge  produced  amounted  to  22  cu.  yds.  per 
million  gallons,  representing  77.8  per  cent,  of  the  suspended 
organic  matter.  This  is  drawn  off  by  a  floating  arm  and  raised 
by  Shone  ejectors  to  a  storage  tank  where  30  to  50  Ibs.  of  lime  per 
thousand  gallons  is  added  in  the  form  of  milk  of  lime.  15  or  20 
per  cent,  of  the  supernatant  liquid  is  drawn  off  to  sand  niters  and 
the  heavy  sludge  pumped  under  a  pressure  of  80  Ibs.  per  square 
inch  to  filter  presses.  When  pressed,  this  produces  3.69  tons  of 

1  Rep.  Purification  of  Sewage  and  Water,  1890,  p.  786.     An.  Rep.,  1908,  p.  457. 

2  An.  Rep.  Supt.  of  Sewers,  1909-10. 

3  There  are  six  primary  tanks,  operated  in  series,  100  ft.  X66  2/3  ft.  X  7  ft.  in  size  with 
a  capacity  of  350,000  gallons  and  10  secondary  tanks,  operated  in  parallel,  166  2/3  ft. 
X40  ft.  X7  ft.  in  size  with  a  capacity  of  350,000  gallons. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    231 


sludge  cake  per  million  gallons,  which  is  taken  by  cars  3/4  of  a 
mile  and  dumped  on  low-lying  ground. 

It  has  not  been  found  economical  to  reduce  the  moisture  by 
pressing  as  low  as  60  per  cent.,  but  rather  to  let  the  cakes  dry  on 
the  dump.1 

The  results  of  precipitation  are  shown  by  analysis  as  follows. 

RESIDUE  ON  EVAPORATION 


Parts  pel 

*  million 

Per  cent. 

A  ve.  sewage 

Ave.  effluent 

removed 

Total: 
Total  

789 

697 

11.7 

Dissolved  

545 

637 

-16.9 

Suspended  
Volatile: 

244 

60 

75.4 

Total  .  .  :  
Dissolved    . 

367 

228 

300 
271 

18.3 
—  18  9 

Suspended  
Fixed: 

139 

29 

79.1 

Total  

422 

397 

59.2 

Dissolved  

317 

366 

-15.4 

Suspended  

105 

31 

70.5 

TABLE  XXIV 

RESULTS  OF  CHEMICAL  PRECIPITATION  AT  WORCESTER, 
MASSACHUSETTS 


Ten  years,  1901-10 

Year  ending 

Nov.  30,  1910 

Maximum 

Minimum 

Average 

Moisture  in  wet  sludge,  per  cent  91  .80 

94.75 

•90.20 

92.39 

Moisture  in  sludge  cake,  per  cent  

68.4 

73.0            67.8 

69.4 

Tons  solids  per  million  gallons  sewage 

1  .  17 

1  .  74     j          0  .  99 

1.37 

treated. 

Pounds   lime   added   per   1000  gallons 

53.5 

53.5 

20.0 

40.7 

sludge. 

Cost  of  operation: 

Per  million  gallons  sewage  

$4  .  53 

$6.33 

$3.85 

$5.05 

Per  ton  solids  

$3.88 

$4.33 

$3.39 

$3.74 

1  An.  Rep.  Supt.  Sewers,  1898-99. 


232  SEWAGE  SLUDGE 

Providence,  R.  I. — The  average  volume  of  sewage  treated  per 
day  for  the  year  1910  was  14,652,329  gallons.  The  total  sewage 
produced  was  about  15  million  gallons  per  day,  and  the  popula- 
tion served  was  199,000,  making  75  1/2  gallons  of  combined 
sewage  per  capita.  This  contains  wastes  from  wool-washing, 
dyeing,  bleaching  and  jewelry  works,  and  its  analysis  shows  the 
following  albuminoid  ammonia  in  parts  per  million:  Suspended, 
4.84;  soluble,  4.68.  Total,  9.52. 

This  was  treated  with  485.5  Ibs.  of  lime  per  million  gallons, 
removing  48.32  per  cent,  of  the  organic  matter  (as  albuminoid 


FIG.  40. — Sludge  presses,  Providence. 
(Courtesy  of  Mr.  O.  F.  Clapp,  City  Engineer.) 

ammonia),  and  82.64  per  cent,  of  the  suspended  matter.  The 
sludge  amounted  to  23.15  cu.  yds.  per  million  gallons  and  con- 
tained 92.07  per  cent,  moisture. 

Sedimentation  takes  place,  first,  in  4  primary  tanks  11.87  ft. 
deep  and  then  in  16  secondary  tanks  8.67  ft.  deep,  whose  aggre- 
gate effective  capacity  is  11.13  million  gallons.  About  93  per 
cent,  of  the  sludge  is  removed  by  the  primary  tanks,  so  that  at 
times  it  has  been  possible  to  omit  the  use  of  the  secondary  series. 
The  sludge  is  forced  under  a  pressure  of  about  70  Ibs.  per  square 
inch  to  18  presses  holding  from  43  to  54  plates,  each  36  in. 


SLUDG1-:  TREATMENT  IN  THE  UNITED  STATES    233 

square,  by  which  it  is  pressed  into  cake  from  3/8  to  1  1/4  in. 
thick.  About  ">.()!  tons  of  cake  per  million  gallons  of  sewage  are 
produced,  or  0.24  ton  per  cubic  yard  of  wet  sludge.  The  pressed 
cake  contains  72.4  per  cent,  moisture.  Forty-seven  pounds  of 
lime  per  thousand  gallons  were  added  to  the  sludge  before  press- 
ing. Two  men  working  together  will  remove  about  50  tons  of 
cake  from  the  presses  in  10  hours.  Ordinarily,  eight  men  are 
employed  on  16  presses. 

The  cost  of  chemical  precipitation  in  1910  was  $3.11  per 
million  gallons  of  sewage,  and  that  of  sludge  disposal,  $4.06, 
making  the  total  cost  $7.17.  The  sludge  pressing  cost  $2.62  per 
ton  of  solids. 

Alliance,  Ohio.1 — About  1.6  million  gallons  of  sewage  per  day 
were  received  by  a  separate  system  of  sewers  from  6500  persons 
in  1907  and  passed  though  2-in.  and  1/4-in.  bar  screens  to  3 
precipitation  tanks,  80  ft.  X40  ft.  X6  ft.  in  size,  having  a  total 
capacity  of  420,000  gallons.  The  sludge  is  pressed,  reducing  the 
moisture  from  88  per  cent,  to  47  per  cent.,  1500  tons  of  wet 
sludge  furnishing  60  tons  of  pressed  cake,  or  2.5  tons  of  cake  per 
million  gallons  of  sewage.  The  sludge  cake  is  usually  taken 
away  by  farmers  for  fertilizer. 

The  cost  of  operation  was,  in  1906,  45  cts.,  and  in  1907,  55  cts. 
per  capita. 

Canton,  Ohio.1 — About  2  1/2  million  gallons  of  domestic 
sewage  were  contributed  daily  in  1908  by  a  population  of  about 
23,500.  After  passing  a  screen  rack  2  ft.  6  in.  X  4  ft.  2  in.  in 
size,  inclined  20  degrees  to  the  vertical,  and  composed  of  3/1-6 
in.  Xl  1/4  in." bars,  set  7/8  in.  apart,  the  sewage,  which  contains 
but  43  parts  per  million  of  suspended  matter  on  account  of  the 
large  proportion  of  ground  water,  is  treated  with  about  13.6 
grains  of  lime  per  gallon  (1  ton  per  million  gallons)  on  week  days, 
and  half  this  amount  on  Sundays.  It  then  passes  to  a  series  of 
4  tanks  100  ft.  X50  ft.  Xo  ft.  in  size,  having  a  total  capacity  of 
700,000  gallons,  for  precipitation.  The  surface  velocity  was 
measured  in  the  first  of  these  tanks  and  found  to  be  41  ft.  per 
minute.  In  the  other  three  tanks  it  was  27.8  ft.  per  minute. 

The  period  of  retention  was  6.7  hours.  The  first  two  tanks 
are  cleaned  3  times  a  week,  the  last  two  2  times  a  week.  The 
sludge  is  pumped  to  neighboring  fields  and  plowed  under,  but 
this  mode  of  disposal  has  not  proved  satisfactory. 

1  Rep.  State  Board  of  Health,  1908. 


234 


SEWAGE  SLUDGE 


The  cost  of  operation  in  1901  was  $3850,  which,  at  2  1/2 
million  gallons  per  day,  would  be  $4.22  per  million  gallons. 

In  winter,  with  plain  sedimentation,  13.7  cu.  yds.  of  wet  sludge 
were  removed  in  this  way  per  million  gallons  of  sewage,  while  in 
warm  weather,  with  chemical  precipitation,  the  amount  removed 
was  14.3  cu.  yds.  per  million  gallons.  The  suspended  solids, 
which  amounted  to  about  86  parts  per  million,  were  reduced  by 
about  one-half  in  each  case,  and  with  chemical  precipitation 
the  total  organic  matter  was  reduced  by  about  the  same  amount. 
The  greater  part  of  the  sedimentation  took  place  in  the  first 
tank. 

The  results  of  several  analyses  are  given  in  the  following 
table: 

TABLE  XXVI 

REMOVAL  OF  SUSPENDED  MATTEK  AT  CANTON 


Parts  per  million 

Per  cent. 

Tons  of 

removed 

dry  solids 

removed 

Total 

Volatile 

per 

million 

Total 

Volatile 

gallons 

Sewage 

Effluent 

Sewage 

Effluent 

sewage 

Plain  sedimentation: 

• 

Jan    16    1907 

83 

41 

42 

29 

51 

31 

0.175 

Feb   26   1907 

124 

61 

62 

28 

51 

55 

0.263 

Chemical  precipitation  : 

Aug.  9,  10,  1906  

43 

42 

31 

30 

2 

3 

0.004 

July  17,  1907  89 

51 

47 

40 

45 

17 

0.158 

July  18,  1907  118 

58 

65 

30 

51 

54 

0.250 

1                 | 

Although  about  50  per  cent,  of  the  organic  matter  is  removed, 
the  effluent  is  unstable  and  not  entirely  satisfactory,  and  the 
extra  cost,  due  to  the  use  of  chemicals,  does  not  appear  to  be 
justified  by  the  results. 

The  annual  cost  of  operating  the  works  is  about  15  cts.  per 
capita  of  population. 

White  Plains,  N.  Y.1— This  plant,  operated  under  the  patent 
process  of  J.  J.  Powers,  will  soon  be  discontinued  owing  to  the 
construction  of  the  Bronx  Valley  trunk  sewer.  In  1907  there 
were  nearly  14,000  persons  contributing  about  850,000  gallons 
of  strictly  domestic  sewage  daily. 

1  Rep.  N.  Y.  State  Bd.  of  Hlth.,  1907. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    235 

This  passed  through  a  vertical  screen  of  1/8  in.  X  1/2  in.  bars 
4  ft.  long  spaced  1  in.  center  to  center,  to  a  sedimentation  cham- 
ber 45  ft.  X  24  ft.  in  size;  5  or  6  barrels  (1200  to  1300  Ibs.)  of 
lime  were  added  daily  with  a  varying  amount  of  perchloride  of 
iron — frequently  a  carboy  (140  Ibs.)  a  day. 

When  removing  sludge,  once  a  week,  the  tank  is  disinfected 
with  chlorine,  as  described  hereafter  for  the  East  New  York 
plant. 

The  sludge,  amounting  to  about  35  cu.  yds.  per  week,  or  5.9 
cu.  yds.  per  million  gallons  of  sewage,  is  pumped  to  2  drying 
beds  having  an  area  of  about  3600  sq.  ft.  After  drying,  a  small 
part  of  this  is  utilized  as  a  fertilizer. 

The  annual  cost  of  the  process  for  material  was: 


Coal,  145  tons  at  $5 . 25 

Perchloride  of  iron,  300  carboys  of  140  Ibs.  at  $0.0275. 

Lime,  2,240  bbls.  at  1 .40 

Vitriol,  40  carboys  of  140  Ibs.  at  $0.0275 


$     761.25 

1,155.00 

3,136.00 

154.00 


$5,206.25 


or  $16.78  per  million  gallons  treated. 

Brooklyn,  N.  Y.1 — The  Borough  of  Brooklyn,  New  York, 
maintains  4  chemical  precipitation  plants,  employing  the  process 
patented  by  J.  J.  Powers.  Two  of  these  are  at  Coney  Island, 
one  near  Sheepshead  Bay,  and  the  third  and  largest  at  East 
New  York.  Being  similar  in  principle,  the  latter,  only,  will  be 
described. 

The  sewage,  amounting  in  1907  to  about  12  million  gallons 
per  day  from  a  population  of  about  100,000  persons,  including 
the  surface  drainage  from  3200  acres,  is  first  dosed  with  lime 
to  the  amount  of  5  bbls.  per  million  gallons  and  then  enters,  in 
parallel,  two  sedimentation  channels  16  ft.  wide  X  7  ft.  deep  X 
350  ft.  long.  From  these  it  passes  to  a  well  40  ft.  in  diameter 
from  which  it  is  pumped  to  an  outfall  flume. 

For  from  36  to  48  hours  before  cleaning  out  the  tanks  the 
sludge  is  disinfected  with  chlorine  generated  from  108  Ibs. 
sulphuric  acid,  64  Ibs.  common  salt  and  48  Ibs.  manganese  dioxide. 
The  sludge  is  then  pumped  on  to  shallow  lagoons  excavated  near 
the  plant  and  dried. 

1  Rep.  Metropolitan  Sewerage  Com.  of  N.  Y.,  1910,  p.  259. 


236 


SEWAGE  SLUDGE 


TABLE  XXVII 

RESULTS  OF  ANALYSES  OF  SEWAGE  MADE  DEC.  11,  1907,  AT  EAST  NEW 
YORK  DISPOSAL  PLANT  BY  DR.  D.  D.  JACKSON.     PARTS  PER  MILLION 


Raw 

Effluent 

Dissolved 

Suspended 

Total 

Dissolved 

Suspended 

Total 

Total  solids  

441 

136 

577 

454 

134 

588 

Loss  on  ignition  1          176 

115 

291 

110 

98 

208 

Fixed  solids  j          265 

21        !     286 

344 

36 

380 

Fats  and  fatty  acids  

397 

234 

At  the  four  above-mentioned  plants  there  were  said  to  be 
produced  in  1907  33  1/3  cu.  yds.  of  sludge  per  million  gallons 
of  sewage. 

There  were  used  for  precipitation  1.15  bbls.,  or  263  Ibs.  of 
lime  and  133  Ibs.  of  perchloride  of  iron  per  million  gallons  of 
sewage,  and  for  disinfection  of  sludge  there  were  used: 


Lbs.  per  million  gal- 
lons sewage 

Lbs.  per  cubic  yard 
sludge 

Sulphuric  acid  

2  8 

084 

Salt  
Oxide  of  manganese 

1.6 

1  2 

.048 
036 

Regarding  the  efficacy  of  these  plants,  Dr.  D.  D.  Jackson 
states : 

"The  process  of  purification  has  not  materially  reduced  either 
the  suspended  matters  or  matters  in  solution.  ***** 
The  effluent  is  *  *  putrescible  at  the  end  of  24  hours." 


VIII.     THE  DISPOSAL  AND  UTILIZATION  OF  SLUDGE 

1.  Disposal  of  Night  Soil  on  Farms 

The  most  primitive  as  well  as  a  most  effective  method  of 
utilizing  sludge  is  by  its  direct   application  to  the  land   as  a 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    237 

fertilizer.  By  the  removal  of  fecal  matter  from  cesspools  before 
it  has  been  diluted  with  any  large  volume  of  water,  the  processes 
of  sedimentation  and  separation  are  avoided,  although  for  other 
reasons  the  use  of  cesspools  is  not  to  be  advocated.  This  method 
of  disposal  has  been  employed  up  to  the  present  time  on  such  a 
large  scale  in  Baltimore,  Md.,  that  a  brief  description  is  given 
here. 

A  contract  is  entered  into  with  the  city  by  which  the  right  is 
secured  to  charge  the  householder  a  certain  sum  for  the  removal 
of  night  soil.  This  is  drawn  by  suction  with  "odorless  excava- 
tors" from  the  cesspool  when  it  becomes  necessary,  and  con- 
veyed to  barges  holding  450  loads  of  six  barrels  (about  200 
gallons  net)  each,  for  which  the  contractor  operating  the  barges 
receives  25  cts.  per  load  for  disposal.  In  1909,  according  to 
Dr.  James  Bosley,  Commissioner  of  Health,  61,748  loads  were 
removed  in  this  way,  in  addition  to  which  more  or  less  finds  its 
way,  illegally,  by  other  channels  to  farming  land  in  neighboring 
counties.1  The  barges  are  towed  down  the  Patapsco  River, 
chiefly  to  Bear  Creek,  about  8  miles  distant,  where  their  con- 
tents are  pumped  by  specially  designed  pumps  of  large  capacity 
to  lagoons  prepared  for  its  reception  by  the  farmers.  An 
ordinary  lagoon  or  pit  holds  a  scow  load,  or  about  100,000 
gallons,  and  the  operation  of  pumping  occupies  about  two  hours. 
For  this  amount,  which  is  delivered  to  him  as  required,  the  farmer 
paid  the  contractor  several  years  ago  $1.67  per  thousand  gallons. 
The  heavy  sludge  remaining  in  the  scow  is  removed  by  shoveling 
into  carts  and  is  also  taken  by  the  farmer. 

The  pits  are  used  merely  for  storage  until  the  material  is 
required,  when  it  is  bailed  with  long-handled  dippers  into  tank 
carts  and  sprinkled  over  the  fields. 

A  large  variety  of  crops  is  fertilized  in  this  way.  One  farmer 
stated  that  he  had  used  6  barge  loads  of  night  soil  (at  the  rate 
of  4000  gallons  per  acre)  and  35  barge  loads  of  garbage  (also 
handled  in  this  way)  during  the  year  on  150  acres  of  kale,  cab- 
bage, tomatoes,  potatoes  and  spinach.  The  liquid  portion 
appeared  to  be  more  immediately  effective,  but  the  heavier 
portion  produced  a  more  lasting  effect. 

The  odors  in  the  vicinity  of  these  lagoons  are  very  offensive, 
biit,  so  far  as  known,  they  have  not  had  an  unfavorable  effect 
on  the  health  of  those  living  on  the  farms.  The  nuisance  from 

1  Also,  about  2030  houses  are  connected  (1911)  with  storm  water  drains.  Dr.  J.  M.  Bosley. 


238 


SEWAGE  SLUDGE 


flies  has  been  considerable  and  the  possibility  of  conveying 
disease  by  them  should  not  be  forgotten. 

A  more  serious  objection  lies  in  the  illegal  use  of  night  soil  on 
growing  vegetables  before  gathering  for  the  market.  This  is 
very  difficult  to  prevent  on  account  of  the  inaccessible  location 
of  the  farms.  Application  to  the  crops  is  supposed  to  be  made 
several  weeks  (10  in  the  case  of  kale)  before  gathering. 

With  the  introduction  of  sewers  this  system  of  disposal  and 
utilization  will,  of  course,  be  abandoned. 


2.  Dumping  at  Sea 

Disposal  of  sludge  by  dumping  at  sea,  as  practised  at  London, 
Glasgow,  Dublin,  Manchester  and  Salford,  is  almost  unknown 
in  the  United  States.  The  cost  at  several  of  these  places  is  as 
follows: 


TABLE  XXVIII 

COST   OF   TRANSPORTING   SLUDGE   TO   SEA.     ADAPTED   FROM   REPORT    V, 
ROYAL  COMMISSION  ON  SEWAGE  DISPOSAL 


Total  cost  in 

Pont  a 

Place 

Years 

Average 
tons  of 
sludge 
per 
annum1 

Average 
per  cent, 
moisture 
in  sludge 

cents  of  sea 
disposal 
incl.  int.  and 
sinking  fund 
per  ton  of 
sludge1 

Cents 
per  ton 
of  dry 
solid 
matter1 

uents 
per  ton 
of  sludge 
90  per 
cent. 
water1 

Remarks 

Glasgow  

1906-7 

341,600 

86.8 

9.7 

74.0     !     7.42± 

60  year  loan. 

Salford  !  1902-6 

152,320 

79.0 

17.1 

81.5     ;     8.15± 

Heavy  canal 

dues. 

Dublin  1906-7 

128,307 

90.  0± 

9.0 

90.9 

9.09± 

No  harbor 

dues,    short 

distance. 

London  

1903-6 

2,838,080 

92.0 

8.2 

103.0 

10.30± 

Manchester  .... 

1903- 

188,720 

86.  0± 

17.4 

124.7 

12.47± 

Heavy  canal 

4-5-7 

dues. 

Southampton  .  .  1  906-7 

15,624 

90.  0± 

30.5 

305  .  9 

30.59 

By  contract. 

Further  details  are  given  in  the  following  table :: 


1  Tons  referred  to  are  of  2000  pounds. 

2  Fifth  Rep.  Royal  Com.  on  Sew.  Disp.,  p.  167. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    239 
TABLE  XXIX 


Works 

For 
year 

Per  cent, 
moisture 

Cost  per  ton 

Cost  per  million  gallon 

Land 
charges 

Sea 
charges 

Total 

Land 
charges 

Sea 
charges 

Total 

London: 

1907 

1907 

1907-8 
1906-7 

92.04 
91.65 

86.69 

$0.050 
0.046 

0.016 
0.044 

$0.067 
0.067 

0.049 
0.117 

$0.117 
0.113 

0.065 
0.161 

$1.57 
0.78 

0.34 

$2.10 
1.13 

1.06 

$3.67 
2.91 

1.40 

(llasgow: 
Dalmuir  
Manchester  

The  first  three  sludges  are  from  chemical  precipitation  and 
the  last  from  septic  tank  treatment. 

Recently  Providence  has  disposed  of  its  sludge  cake  by  dump- 
ing it  in  Narragansett  Bay  from  a  scow  135  ft.  long X  38  ft.  wide 
Xll  ft.  deep.  This  is  divided  into  6  compartments  and  has  a 
capacity  of  850  cu.  yds.  when  filled  level,  and  1050  cu.  yds. 
when  heaped.  This  is  towed  about  10  miles  down  the  bay  and 
deposited  in  a  depth  of  about  65  ft.  of  water. 

As  already  mentioned,  the  sludge  removed  from  the  deposit 
sewer  of  the  Boston  Main  Drainage  is  taken  by  a  scow  out  into 
Massachusetts  Bay  and  dumped  in  deep  water. 

Disposal  of  sludge  by  dilution  is  also  practised  at  Columbus, 
Ohio,  where  it  is  stored  until  the  river  water  is  in  freshet  or  of 
sufficient  volume  to  render  its  discharge  unobjectionable.  This 
was  also  tried  with  the  sludge  from  the  experimental  tanks  at 
Waterbury,  Conn.,  where  it  was  observed  that  when  diluted  by 
1650  volumes  of  water  in  the  Naugatuck  River,  there  were  no 
apparent  odors  resulting  and  the  mixture  was  non-putrescible. 

3.  Application  to  the  Land 

Direct  application  to  the  land  is  frequently  employed  at  small 
works.  The  ordinary  cost  of  this  is  given  by  Mr.  W.  B.  Ruggles 
as  from  40  to  50  cts.  per  ton  of  the  solid  content  and  the  area 
required  is  from  1  to  2  acres  (2  or  3  in.  deep)  per  1000  tons  of 
sludge.1  Where  trenching  and  burying  is  used,  the  cost  is  about 
9  to  1 4  cts.  per  ton  of  wet  sludge,  or  from  90  cts.  to  $1.40  per  ton 


i  Kinnicutt,  Winslow  and  Pratt. 


240 


SEWAGE  SLUDGE 


of  solids,  exclusive  of  the  cost  of  land.  The  area  required  is 
from  1/4  to  1/2  acre  per  1000  inhabitants/  or  from  0.2  to  0.4 
acres  per  1000  tons  of  sludge. 

At  Mansfield,  Ohio,  the  total  cost  of  disposing  of  1200  cu.  yds. 
of  septic  sludge  on  the  land,  employing  6  men  and  a  horse  at  a 
cost  of  $15  per  day,  was  about  50  cts.  per  cubic  yard.  About 
40  cu.  yds.  were  handled  per  day  of  8  hours.  No  nuisance  is 
experienced  except  during  the  operation  of  emptying  the  tanks, 
when  there  is  a  noticeable  odor. 

At  White  Plains,  N.  Y.,  the  sludge  from  chemical  precipitation 
is  pumped  once  a  week  on  to  land  to  a  depth  of  3  in.  In  about 
7  days  it  dries  sufficiently  to  be  winrowed  and  is  later  wheeled  to 
a  dump.  About  5  cu.  yds.  were  produced  from  a  population  of 
14,000.  Two  sludge  beds  of  1800  sq.  ft.  each  were  used  alter- 
nately. 

Drying  in  lagoons  is  practised  in  Reading,  Pa.,  the  area  required 
being  about  1/4  acre  for  the  4280  cu.  yds.  of  wet  sludge  produced 
in  1910.  No  offensive  conditions  were  noted  during  the  year. 

Experiments  were  conducted  at  the  Philadelphia  testing  sta- 
tion with  drying  in  4  lagoons  8  ft.  X  12  ft.  in  size. 

With  sludge  derived  from  plain  sedimentation,  the  results  were, 
as  follows: 

TABLE  XXX 

RESULTS  OF  DRYING  SLUDGE  IN  LAGOONS.     PHILADELPHIA 


| 
Time  in  days 

I 

Depth  in 
inches 

Per  cent, 
moisture 

Rainfall, 
inches 

Cubic  yards 
sludge  per  acre 

Screened.  .  . 

0 

12.20 

82.8 

0 

1600 

Screened.  .  . 

26 

7.67 

57.0 

0 

1000 

Screened.  .  . 

49 

3.50 

51.6 

0.43 

470 

Screened.  .  . 

0 

13.50 

90.1 

0 

1800 

Screened.  .  . 

62 

7.00 

61.0 

3.14 

950 

Crude  

.  .1              0 

12.00 

88.7 

0 

1600 

Crude  

59 

4.70 

62.8 

2.59 

640 

In  general,  wet  sludge  12  in.  deep  dries  to  about  60  per  cent, 
moisture  in  6  weeks,  leaving  about  0.4  of  the  original  volume  to  be 
removed  from  the  bed  or  lagoon. 


1  Eng.  Rec.,  Vol.  LXIII,  p.  79. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    241 

Sludge  from  the  Emscher  tank  dried  rapidly  and  soon  became 
odorless.  It  was  run  onto  a  bed  under  cover  consisting  of: 

Fine  sand  .  . 
Gravel  .... 

*""  I  Foundation  resembling  nat- 

Gravel 6m.  >  .... 

rtj  .  ural  conditions. 

Broken  concrete 

This  was  underdrained  by  3-in.  perforated  tiles. 

It  was  found  that  12  in.  of  sludge  placed  on  this  bed  in  winter 
was  in  condition  to  be  removed  in  12  days,  although  containing 
68  per  cent,  moisture. 

The  average  time  for  drying  in  the  Emscher  District,  according 
to  Mr.  Charles  Saville,1  is  about  7  days,  but  in  summer  it  is 
sometimes  removed  after  but  2  days,  the  moisture  in  the  dried 
sludge  varying  from  55  to  65  per  cent. 

Experiments  were  made  in  lining  the  lagoons  with  various 
materials:  coarse  sand,  fine  sand,  rice  coal  and  sawdust.  Coal 
and  sawdust  were  favorable  to  subsequent  incineration  while 
sand  was  liable  to  form  clinker.  "The  thick  layer  of  sawdust 
was  more  efficient  than  the  thin,  whereas  the  thick  layer  of  coal 
was  less  efficient  than  the  thin,  and  the  thick  layer  of  sawdust 
was  equally  efficient  to  the  thin  layer  of  coal."  About  the  same 
amount  of  moisture  was  removed  by  sand  and  sawdust,  this 
being  about  75  per  cent. 

At  the  Elmhurst  (Borough  of  Queens,  New  York)  plant 
the  supernatant  water  is  drawn  from  the  tank  at  mid-depth. 
The  sludge  with  the  remaining  roily  liquid  is  then  placed  on  a 
sludge  filter  20  ft.  X  50  ft.  in  area  and  3  ft.  3  in.  deep.  The 
filter  is  under  cover.  It  is  made  of  graded  material  varying  in 
size  from  3  in.  at  the  bottom  to  sand,  with  4  in.  of  combustible 
material — usually  buckwheat  coal — at  the  top.  The  bed  is  un- 
derlain by  a  system  of  2  1/2-inch  steam  pipes  to  facilitate  drying. 

The  heavy  liquid  is  delivered  to  the  surface  of  the  bed  by  a 
12-in.  pipe  and  a  trough.  The  filtrate  passes  by  underdrains  to 
a  pump  well  and  the  de-watered  sludge  is  scraped,  with  the  coal, 
from  the  surface  after  about  3  days'  drying,  and  burned  under 
boilers.2 

Tests  made  at  the  Chicago  experimental  plant  showed  that 
sludge  from  plain  sedimentation  containing  90  per  cent,  moisture 

1  Ass't  Eng'r  Emscher  Association,  Eng.  News,  Vol.  LXV,  p.  664. 
*Eng.  Rec.,  Vol.  LII,  p.  87. 
16 


242  SEW  AGE  SLUDGE 

dried  out  to  a  thickness  of  4  in.  with  but  50  per  cent,  moisture  in 
30  days  during  warm  weather. 

At  Brockton,  Mass./  the  dried  sludge  raked  from  the  inter- 
mittent filters  was  first  burned  on  wood  fires.  As  this  caused  a 
nuisance,  it  was  then  (1890)  sold  to  farmers  for  $125,  and  later 
(1901  to  1906)  for  $150  per  annum.  Since  1909  it  has  been  given 
away  so  as  to  secure  a  prompt  removal.  In  1908  the  sludge 
averaged  136,000  gallons  per  day  and  contained  11,177.5  parts 
per  million  of  total  solids.  It  produced  about  3500  tons  of  dry 
sludge.  The  rakings  were  of  the  following  composition: 

Moisture 16 . 22  per  cent. 

Phosphoric  acid 78  per  cent. 

Potassium  oxide 51  per  cent. 

Nitrogen 1 . 45  per  cent. 

Calcium  oxide '    .30  per  cent. 

Insoluble  matter,  sand,  etc 70. 13  per  cent. 

This  is  used  as  a  fertilizer  on  corn,  potatoes,  millet  and  other 
grasses,  but,  with  the  exception  of  corn,  additional  potash  and 
phosphoric  acid  are  required. 

In  general,  the  cost  of  raking  and  removing  the  sludge  from 
intermittent  sand  filters  in  Massachusetts  amounts  to  about 
$3  per  million  gallons  of  the  sewage  applied,  or  from  12  to  30  cts. 
per  capita  of  population. 

In  the  arid  portion  of  the  west,  conditions  are  more  often 
favorable  to  the  direct  application  of  the  raw  sewage  to  the  land. 
According  to  Dr.  W.  F.  Snow,  Secretary  of  the  State  Board  of 
Health  of  California,  some  towns  in  that  state  operating  sewage 
farms  realize  from  $500  to  $5000  a  year  in  the  crops  of  hay, 
walnuts,  potatoes,  alfalfa  and  eucalyptus  wood  produced. 

According  to  Dr.  Voelcker,2  the  yield  of  corn  (wheat,  etc.)  is 
increased  from  10  to  12  per  cent,  by  the  application  of  sewage 
sludge  to  the  extent  of  40  Ibs.  of  nitrogen  to  the  acre,  while 
artificial  fertilizers  of  equivalent  strength  increase  the  yield  from 
16  to  17  per  cent.  The  use  of  sludge  increases,  in  particular,  the 
stem  of  the  plant  and  therefore  the  straw  produced,  but  in  any 
case  its  value  depends  even  more  on  the  amount  of  moisture  and 
the  lime  contained  than  upon  the  percentage  of  nitrogen.  He 
concludes  that  from  a  practical  point  of  view  none  of  the  sewage 

1  Eng.  News,  Vol.  LXII,  p.  251. 

2  Fifth  Rep.  Royal  Com.  on  Sew.  Disp.,  p.  187. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    243 

sludges  used  would  be  worth  10s.  ($2.50)  a  ton  on  the  farm  for 
wheat-growing  purposes. 

The  economical  use  of  sludge  as  a  fertilizer  being  exceptional, 
its  disposal  on  land  is  reduced  to  either  merely  drying  or  burying. 
As  to  the  choice  of  these,  Mr.  George  W.  Fuller  states:1 

1.  That   sludge   drying   beds   are   usually  unsatisfactory  for 
large  plants  and  that  when  they  have  been  used  with  a  moderate 
degree  of  success  this  has  usually  been  during  cool  weather. 

2.  That  the  burial  of  sludge  in  trenches  has  merit  in  the  case 
of  small  installations,  but  that  in  the  case  of  large  plants  this 
cannot  compete  with  the  employment  of  the  Emscher  tank,  the 
product  from  which  is  inoffensive  and  is  therefore  easily  dis- 
posed of. 

4.  Filter-pressing 

Pressing  sludge  is  usually  confined  to  plants  employing  chem- 
ical precipitation  and  where,  therefore,  there  are  large  volumes 
of  a  rather  watery  product  to  be  handled. 

At  Worcester  this  process  cost,  in  the  year  ending  November 
30,  1910: 

Per  million  gallons  of  sewage,  $2.76 
Per  thousand  gallons  of  sludge,  $1.20 
Per  ton  of  solids,  $3.50 

The  cake  is  used  for  filling  in  land. 

At  Providence  the  total  cost  of  sludge  disposal  in  1909  was 
$4.22  and  in  1910  $4.06  per  million  gallons  of  sewage,  and  the 
cost  of  sludge  pressing  was  $2.85  and  $2.62,  respectively,  per  ton 
of  dry  solids. 

Aside  from  the  Worcester  and  Providence  plants,  where  the 
cost  data  are  carefully  kept,  the  information  to  be  had  from 
American  practice  is  so  meager  that  the  following  supplementary 
figures  referring  to  sludge  pressing  in  England  are  given. 

In  Leeds,  according  to  W.  W.  Ruggles,2  the  cost  of  pressing 
was,  in  1910,  but  $14,784.61  for  16,0.17  tons  of  dry  solids,  or 
about  92  cts.  per  ton. 

1  Sewage  Disposal  with  respect  to  Offensive  Odors.     M.  I    T.  Congress  of  Technology, 
April,  1911. 

2  Exclusive  of  sewage  beds  and  niters.     Sewage  Sludge  Disposal,  Eng.  Rec.,  Vol.  LXIII, 
p.  79. 


244  SEWAGE  SLUDGE 

Mr.  Ruggles  gives  the  cost  of  cremating  sludge  cake  as  about 
$3  per  ton  of  dry  material,  and  that  of  carting  and  dumping  the 
cake  as  seldom  less  than  60  cts.  per  ton  and  frequently  two  or 
three  times  that  amount,  depending  on  the  haul. 

According  to  the  Royal  Commission  on  Sewage  Disposal,1  the 
cost  of  pressing  sludge  under  ordinary  conditions,  reducing  the 
moisture,  90  to  95  per  cent,  in  the  raw  sludge,  to  about  55  per 
cent,  in  the  pressed  cake,  may  be  taken  as  follows: 

TABLE  XXXI 
COST  OF  PRESSING  SLUDGE  INCLUDING  INTEREST  AND  SINKING  FUND 


Wet  sludge  per  ton       Pressed  cake  per  ton 


For   populations   of   30,000   or  more  and  j  13.2  cts.  to  15.6  cts.     59.7  cts.  to  70.6  cts. 
ordinary  sewage. 


For  populations  less  than  30,000  and  where, 
on  account  of  septic  or  greasy  sludge,  5  to 
20  per  cent,  lime  had  to  be  added. 


18.1  cts.  to  28.0  cts. 


81.4  cts.  to  $1.249 


According  to  Santo  Crimp,  if  the  moisture  is  reduced  by  press- 
ing to  50  per  cent.,  the  product  from  each  inhabitant  will  equal 
2  cwt.,  or  0.112  ton,  per  annum  after  efficient  chemical  precipi- 
tation. 

Pressed  sludge  cake  weighs,  according  to  Rideal,  82/3  tons 
per  million  gallons  of  sewage,  and  the  moisture  can  be  reduced 
from  50  per  cent,  to  12  per  cent,  by  air  drying. 

The  value  of  this  air-dried  sludge  he  estimates  for  different 
English  plants  as  follows: 

From  $2.48  (Birmingham  using  lime,  and  Windsor  employ- 
ing the  Hilles  process)  to  $5.90  (Coventry  using  sulphate  of 
alumina)  per  ton  of  2000  Ibs.  The  dried  sludge  at  Aylesbury, 
where  the  ABC  process  is  employed,  is  valued  at  $7.10  per  ton. 

5.  Drying  with  Centrifugal  Machines 

Drying  by  centrifugal  machines  has  hardly  been  attempted  in 
the  United  States.  While  admitting  the  excellent  results  ob- 
tained by  the  Schaefer-ter  Meer  machine  abroad,  its  high  first 
cost  has  prevented  its  introduction  into  this  country  up  to  the 
present  time. 

1  Fifth  Rep.  Royal  Com.  on  Sew.  Disp.,  p.  170. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    245 

A  centrifugal  dryer  of  more  simple  construction  is  used  al 
Reading,  Pa.,  however,  for  de-watering  the  material  received 
from  the  rotary  screen.  The  sludge  is  delivered  with  89.5  per 
cent,  moisture  by  a  screw  conveyer  to  canvas  bags.  These  are 
placed  by  hand  in  the  hydro-extractor,  which  is  about  6  ft.  in 
diameter  and  31/2  ft.  high.  On  removal  the  moisture  has  been 
reduced  from  89.5  to  73  per  cent.,  19.6  per  cent,  of  the  product 
being  volatile,  and  the  weight  has  been  reduced  from  62-70  to 
31-351bs.  per  cubic  foot.  The  material  taken  from  the  machine 
is  burned  under  the  boilers  of  the  sewage  pumping  station. 

The  manual  labor  required  in  the  operation  is  a  serious  objec- 
tion to  this  type  of  dryer  in  connection  with  large  plants. 


FIG.  41. — Centrifugal  sludge-drying  machine  at  Frankfort,  Germany. 
(Courtesy  of  The  Lathbury-d'Olier  Co.,  Philadelphia.) 


In  Bradford,  England,  the  cake  from  sludge  presses  is  heated 
in  a  rotary  drier,  which  reduces  the  moisture  from  33  to  about 
9  per  cent.,  leaving  the  dried  product  in  a  form  suitable  for 
shipping.  This  is  said  to  find  a  ready  market  at  $2.17  per  ton 
and  has  proved  so  profitable  that  similar  machines  are  to  be 
installed  at  the  sewage  treatment  plant  at  Dublin. 

The  cost  of  producing  the  dried  product  for  use  as  a  filler  for 


246  SEWAGE  SLUDGE 

fertilizers  under  American  conditions  is  estimated  by  Mr.  Ruggles 
as  follows: 

Cost  per  ton 

Filter  pressing $1 . 00 

Drying .35 

Grinding 16 

Bagging 15 

Total $1 . 66 

While  its  value  as  a  fertilizer,  which  has  been  separately 
estimated  at  $6.76  and  $10.79  per  ton,  is  assumed  to  be  at  least 
$4,  leaving  a  profit  of  $2.34  per  ton.1  The  cost  per  cu.  yd.  of 
dried  sludge  at  Hanover  has  been  estimated  at  36  cents  and  in 
America  would  probably  be  more  than  double  this  amount.2 

Kinnicutt,  Winslow  and  Pratt  give  the  probable  cost  of  dry- 
ing by  centrifugal  machines  as  from  5  to  7  cents  per  cu.  yd.  of 
wet  sludge. 

With  regard  to  the  Schaefer-ter  Meer  machines,  the  following 
data  are  from  the  operation  of  the  4  units  installed  at  Hanover: 

Dried  material  produced  per  unit  per  day 26-39    cu.  yds. 

Dried  material  produced  per  million  gallons  sewage 3 . 3-4 . 9  cu.  yds. 

Cost  of  operation: 

Per  unit  per  day $12 . 85 

Per  capita  per  annum .02 

Per  million  gallons  sewage 1 . 62 

Per  cubic  yard  wet  sludge .  07-.  10 

Per  cubic  yard  dried  sludge .  33- .  50 

6.  Recovery  of  Calorific  Value 

No  attempt  has  been  made  to  utilize  the  latent  calorific  value 
of  sludge  on  a  large  scale  in  the  United  States,  but  some  experi- 
ments have  been  made  in  this  direction  by  the  Massachusetts 

1  Estimate  made  by  a  "  well-known  laboratory  in  New  York." 

Ammonia,  2.6  per  cent $6.52 

Equivalent  of  bone  phosphate,   .66  per  cent .07 

Potash,  .24  per  cent .' .17 

$6.76 
Estimate  on  sample  furnished  The  American  Fertilizer  of  Philadelphia. 

Nitrogen,  52 .2  Ibs.  at  20  cts $10 .44 

Phosphoric  acid,  2.2  Ibs.  at  4  cts 09 

Potash,  6.2  Ibs.  at  4  .25  cts .26 

$10.79 

2  Rep.  Sew.  Disp.  Chicago.     Geo.  M.  Wisner,  1911. 


SLUDGE  TREATMENT  IN  THE  UNITED  STATES    247 

State  Board  of  Health,1  by  the  city  of  Worcester,  Mass.,2  and 
by  the  City  of  Philadelphia.3 

In  1898  the  Massachusetts  State  Board  of  Health  demon- 
strated that  gas  was  evolved  from  the  sludge  rather  than  from 
the  soluble  contents  of  sewage. 

In  the  year  1900  the  following  volumes  of  gas  were  produced 
in  a  septic  tank,  illustrating  clearly  the  effect  of  temperature. 

TABLE  XXXII 
GAS  PRODUCED  IN  SEPTIC  TANK 


April  21 

May  2  to 

July  10  to 

Oct.  4  to 

to  May  1 

May  22 

July  20 

Oct.  6 

Average  hours  storage  

28 

21 

28 

23 

51 

52 

74 

65 

Cubic  feet  gas  per  1,000,000  gallons  of 

6100 

8400 

11300 

6000 

sewage  passed. 

Cubic  feet  gas  per  1000  gallons  of  tank 

5.3 

9.5 

9.5 

5.3 

capacity. 

Cubic  feet  gas  per  cubic  foot  sludge  in  tank 

0.71 

1.27 

1.27 

0.71 

To  illustrate  the  effect  of  varying  composition,  the  amount  of 
gas  obtained  from  the  fermentation  of  different  sludges  was 
determined. 

TABLE  XXXIII 
AMOUNT  OF  GAS  PRODUCED   BY  FERMENTATION  OP  DIFFERENT  SLUDGES 


Per  cent. 

Centimeters  of  gas  formed  per 

Source 

Days 

organic 
matter  in 

gram  of 

sludge 

Sludge 

Organic  matter 

Tannery  sewage  

61 

51 

0.00 

0.00 

Lawrence  sewage  

26 

84 

0.34 

0.40 

Lawrence  sewage  
Septic  tank  .  . 

21 
30 

78 
46 

5.80                       7.45 
4.14                       9.00 

Septic  tank  gas  was  found  to  be  composed  principally  of 
methane,  carbon  dioxide  and  nitrogen.  The  methane  varied 
from  28.7  to  79.0  per  cent.  When  obtained  from  the  fermenta- 

1  Rep.  Mass.  St.  Bd.  Hlth.,  1908,  p.  492,  et  seq. 

2  Eng.  News,  1892. 

3  Rep.  Bureau  of  Surveys,  comprising  work  at  the  sewage  experiment  station  at  Spring 
Garden,  Philadelphia,  1910,  p.  191,  et  seq. 


248 


SEWAGE  SLUDGE 


tion  of  sludge,  the  methane  varied  from  79  per  cent,  in  the  case 
of  septic  sludge  to  2  per  cent,  in  the  case  of  ordinary  sewage 
sludge. 

TABLE  XXXIV 

V 

COMPOSITION  OF  GAS  PRODUCED 


Per  cent. 

Source 

C02 

CH4 

N 

Septic  tank  A  

3.4 

79.0 

16.0 

Septic  tank  B  

42.2 

37.5 

19.0 

Andover  septic  tank 

9  8 

28  7 

61  0 

Sludge  from,  regular  sewage  .... 

28.7 

1.8 

69.5 

Sludge  from  septic  tank  

11.7 

75.9 

12.4 

Experiments  were  begun  in  1908  on  the  distillation  of  gas 
from  sludges  of  different  kinds.  The  average  volumes  produced 
were  as  follows: 

TABLE  XXXV 
GAS  PRODUCED  PER  TON  OF  DRY  SLUDGE 


From  settled  sewage  sludge 6600  cu.  ft. 

From  chemically  precipitated  sewage  sludge,  j  8100  cu.  ft. 

From  septic  sludge j  4900  cu.  ft. 

From  peat |  8400  cu.  ft. 

From  soft  coal !  8600  cu.  ft.  to  12,900  cu.  ft. 


While  the  composition  of  the  gases  depended  much  on  the 
source  of  the  material,  those  from  sludge  contained,  in  general, 
more  CO2  and  CO  and  of  "the  so-called  illuminants"  than  those 
derived  from  coal,  while  the  H  and  CH4  were  less  in  quantity. 
The  resulting  coke  amounted  to  from  45  to  65  per  cent,  of  the 
weight  of  the  dry  sludge  and,  although  containing  much  mineral 
matter,  could  no  doubt  be  burned  as  fuel.  Analyses  of  this 
showed  from  1.1  to  1.7  per  cent,  of  available  P2O5  and  about 
22  per  cent,  of  the  nitrogen  in  the  sludge.  "Much  of  the  fats 
*  *  *  *  distilled  over  with  the*  tars.  *  *  *  This  by- 
product could  readily  be  disposed  of  by  mixing  it  with  the  coke 


SLVDdK  TREATMENT  IN  THE  UNITED  STATES    249 


and  burning,  or  if  it  were  formed  in  sufficient  amounts  it  could 
be  burned  directly,  in  the  same  manner  as  water  gas  tars  are 
utilized." 

TABLE  XXXVI 

ANALYSES  OF  SLUDGES  USED,  PER  CENT.  OF  COKE  FORMED  AND  AMOUNT 
OF  NITROGEN  IN  COKE 


Source 

Composition  of  sam- 
ple before  distillation 
per  cent. 

Per 
cent, 
coke 
produced 

Per  cent.  N  (by 
wt.  of  total 
sludge) 

Per 
available 
P205 
in  coke 

Total 

N 

Loss  on 
ignition 

Fats 

Found 
in  coke 

As  NH3 
in  washer 

Lawrence  (settled  sewage)  . 
Andover  (settled  sewage)  .  . 
Clinton  (settled  sewage)..  . 
Brockton  (settled  sewage) 
Worcester  (chem.  precip.) 
Septic  tank 

3.36 
2.14 
2.36 
1.76 
1.19 
2.46 
2.54 

36.8 
46.6 
74.4 
46.6 
44.5 
47.9 
92.0 
96.8 

12.8 
27.5 
7.7 
6.2 
3.2 
8.3 

63.5 
59.5 
44.5 
60.5 
54.0 
68.5 
49.0 
77.3 

.11 
.67 
.72 
.94 
.09 
.27 
.70 

.586 
.226 
.404 
.137 
.544 
.497 
.700 
222 

1.33 
1.33 
1.44 
1.17 
1.67 
1.15 
0.31 

Peat 

Soft  coal  (aver,  of  4  kinds 
steam  and  gas  coal). 

TABLE  XXXVII 

GASES   PRODUCED    BY   DESTRUCTIVE    DISTILLATION    OF   SEWAGE    SLUDGE 


Source 

Cu.  ft.  gas 
per  ton  of 
sample 

C02 

Illumi- 
nants 

o 

CO 

H 

CH4 

N 

Lawrence  (settled  sewage)  

4900 

4.4 

2.2 

0.3 

30.7 

34.9 

18.6 

9.1 

Andover  (settled  sewage)  '         6400 

7  4 

15.1 

0  6 

14.3 

22.9 

34.3 

5  4 

Clinton  (settled  sewage)  9100 

8.3 

6.7 

0  0 

?0  4 

33  9 

24.5 

7  0 

Brockton  (settled  sewage)  

6000 

16.5 

21.4 

0.2 

10.3 

22.6 

29.1. 

0.2 

Worcester  (chem    precip  )                             8100 

14  2 

•"•* 

4  9 

0  ? 

29  8 

32  6 

16  2 

2  2 

Septic  tank                                                        4QOO 

7  5 

1  0 

0  1 

24  '3 

44  0 

13  0 

10  2 

Peat  .  .                .... 

84nn 

39  0 

4  7 

0  2 

11   O 

28  0 

17  1 

0  0 

Soft  coal  10200 

I 
i    1.6 

2.0 

0.1    5.2 

62.3 

25.7 

3.2 

Illuminating  gas  (Lawerence)  

3.4          9.1 

0.021.542.5  19.7 

3.8 

The  Worcester  experiments  referred  to  were  made  in  1891 
and  consisted  in  burning  45  tons  of  sludge  containing  46  per 
cent,  moisture  with  the  aid  of  3  cords  of  wood.  The  total  cost 
of  its  disposal,  including  the  manual  labor  of  collecting  the 
sludge  from  the  beds  and  conveying  it  to  the  furnace,  was  $3 
per  ton  of  dry  sludge. 


250 


SEWAGE  SLUDGE 


At  the  Philadelphia  sewage  experiment  station  wet  sludge 
was  mixed  with  an  equal  weight  of  rice-size  anthracite  coal. 
The  resulting  mixture  was  1.57  times  the  volume  of  the  sludge 
and  its  specific  gravity  1.29.  The  percentage  of  moisture  was 
reduced  in  this  way  from  91  per  cent,  to  48  1/2  per  cent.  After 
placing  in  a  sludge  lagoon  to  a  depth  of  12  in.  and  drying  24 
hours,  this  was  reduced  to  about  27  1/2  per  cent.,  and  in  9  days 
to  22  1/2  per  cent.,  the  temperature  being  about  37°  F. 

The  result  of  the  mixing  is  shown  in  the  followng  table: 


Constituents 

Per  cent. 

Lbs.  per  cu.  yd. 

Moisture                                 ...             .      . 

45.5 
50 
4.5 

1,069 
1,175 
106 

Coal 

Dry  residue  of  the  sludge 

100. 

2,350 

Each  cubic  yard  of  wet  sludge,  after  drying,  with  1760  Ibs. 
of  coal,  produced  one  ton  of  the  dried  mixture  delivered  at  the 
boiler  house  for  fuel. 

The  British  thermal  units  contained  in  the  materials  used 
were: 

In  the  coal  as  received 12065 

In  the  sludge  as  burned 1216  to  2165 

TABLE  XXXVIII 
RESULTS  OF  BURNING  AIR-DRIED  SLUDGE  WITH  COAL 


Weight  of  sludge  broken  to  2-in.  size,  per  cu.  yd.  in  Ibs, 

Percentage  of  water  in  sludge 

Percentage  of  dry  residue,  volatile 

Lbs.  dry  residue  in  sludge  used 

Lbs.  volatile  matter  in  sludge  used 

Lbs.  coal  burned  with  sludge 

Lbs.  wet  sludge  burned  per  minute 

Lbs.  volatile  matter  burned  per  minute 

Lbs.  dry  residue  burned  per  minute 

Lbs.  of  coal  burned  per  pound  of 

Wet  sludge 

Dry    residue 

Volatile  matter 


710    to  1015 
15.3-  to   40.2 
24.5   to   30 


168 
48 
192 
2.66 

.  555  to 
2.18  to 


233 
70 

285 
4 


.68  to 
.817  to 
.  233  to 


15 

.705 

2.47 

.895 
.945 
.25 


SLUDGE  TRKA  TMENT  IN  THE  UNITED  STATES    251 


The  experiment  demonstrated  that  it  was  possible  to  burn 
sludge  in  this  way  under  boilers,  but  the  degree  of  economy 
effected  was  not  determined. 

Samples  were  then  taken  of  air-dried  screened  sewage  sludge, 
crude  sewage  sludge  and  Emscher  tank  sludge  and  mixed  with 
equal  weights  of  pea  coal,  and  of  wet  sludge  mixed  with  equal 
weights  of  rice  coal,  but  the  moisture  to  be  evaporated  inter- 
fered with  realizing  their  full  caloric  value.  The  small  coal  con- 
sumption it  was  believed,  however,  would  frequently  justify  the 
employment  of  this  process  in  connection  with  sludge  disposal. 

TABLE  XXXIX 

RESULTS  OF  TESTS  OF  FUEL  VALUE  FOR  STEAM  PRODUCTION  OF  MIXTURE 
OF  SLUDGE  AND  COAL 


Per  cent, 
wet  sludge 
in  mixture 

Fuel  prior  to  burning 

Lbs.  water 
evap.  per  Ib. 
fuel 

Eqivalent 
evaporation  from 
and  at  212°  F  . 
per  Ib.  of  fuel 

Moisture 

b.t.u. 

Rice  coal  and  wet 

12.4-     2.75-3.87 

10700-11252 

4.20-4.50 

4  ,  60-4  .  92 

sludge. 

22.9 

Pea  coal  and  dry  |        50             1.1  5-2  .  03 

8832-8875 

2.67-3.32 

2.92-3.63 

sludge. 

CHAPTER  IX 
SUMMARY  AND  CONCLUSIONS 

In  selecting  the  best  method  for  removing  the  solid  matter 
suspended  in  sewage  we  must  consider  the  kind  of  subsequent 
treatment,  if  any,  it  is  to  receive.  If  the  effluent  is  to  be  put 
through  sprinkling  niters  or  contact  beds  or  if  it  is  to  be  applied 
to  the  land  it  should  be  delivered  in  as  fresh  a  condition  as 
possible  and  the  coarser  particles  should  be  removed  by  screens, 
scum  boards,  grit  chambers  or  a  combination  of  some  such  de- 
vices. Otherwise  the  process  will  be  more  offensive  and  the 
filter  beds  more  likely  to  clog.  If  it  is  to  be  discharged  into  a 
stream,  too  coarse  material  should  be  removed  as  causing  de- 
posits on  the  bottom  or  an  offensive  appearance  of  the  surface 
of  the  water.  If,  however,  it  is  to  be  utilized  on  account  of  its 
calorific  or  fertilizing  properties  the  sludge  from  plain  sedi- 
mentation or  Dortmund  tanks  or  that  from  fine  screens  is  pref- 
erable to  the  more  completely  mineralized  product  from  septic 
or  Emscher  tanks.  If  septic  tanks  are  employed  the  grit  need  not 
ordinarily  be  first  intercepted,  .but  may  be  handled  in  connection 
with  the  other  sludge,  but  in  the  case  of  Emscher  or  Dortmund 
tanks  the  grit  is  undesirable,  as  tending  to  clog  the  discharge 
pipe.  Chemical  precipitation  is  sometimes  to  be  preferred  in  the 
case  of  a  very  strong  sewage  or  one  containing  acid  wastes  in 
large  quantity,  or  in  case  it  is  thought  best  to  press  the  sludge  into 
cake.  If  the  sludge  is  to  be  buried,  air  dried,  used  for  filling  in 
land  or  dumped  at  sea  the  septic  and  Emscher  tanks  have  the 
advantage  of  furnishing  a  product  of  small  volume  which  may  be 
readily  handled  with  the  minimum  offense.  Fine  screening 
requires  but  little  room  and  therefore  should  be  considered  where 
land  values  are  high,  but  in  this  case,  as  well  as  in  plain  sedi- 
mentation, the  resulting  detritus  or  sludge  contains  so  large  an 
amount  of  moist  organic  matter  that  its  prolonged  storage  is 
objectionable  in  populous  districts.  These  questions  have  been 
so  fully  treated  by  Dr.  Eisner  in  Part  I  that  it  is  unnecessary  to 
dwell  further  on  them  here. 

252 


SUMMARY  AND  CONCLUSION 


253 


Having  decided  on  the  general  method  to  be  employed  the 
results  that  may  be  expected,  based  on  experience  in  the  United 
States,  are  about  as  follows: 

TABLE  XL 

REMOVAL  OF  SUSPENDED  SOLIDS  BY  DIFFERENT  METHODS 
OF  TREATMENT 


Method 

Per  cent. 

Cu.  yds.  sludge 

Per  cent. 

removed 

per  mil.  gal.  sew. 

moisture 

Bar  screens,  spaces  3/4  in.  to  1  in  

2  to  10 

0.1  toO.25 

65  to  75 

Mesh  screens,  spaces  1/4  in.  or  less  

15  to  25 

0.6  to  1.4 

80  to  90 

Grit  chambers  

5  to  10 

0.1  toO.8 

35  to  50 

Plain  sedimentation  

50  to  70 

4  to7 

87  to  93 

Septic  tanks  

50  to  70 

1.5  to3 

80  to  90 

Emscher  tanks  

50  to  70 

1  to  2 

75  to  85 

Chemical  precipitation  

75  to  90 

20  to  25 

86  to  92 

These  figures  are  subject  to  so  great  a  variation,  depending 
on  local  conditions,  that  they  are  merely  given  as  a  guide  to 
indicate  the  limiting  values  under  ordinary  conditions. 


Cubic  Yards  of  Wet  Sludge  containing  100  Ibs.of  Dry  Residue. 
.0123456 


Sp.Gr.1.06 


FIG.  42. — Volumes  of  sludge  with  varying  percentages  of  moisture. 
(Reproduced  from  Report  on  Disposal  of  Sewage,  Philadelphia,  1911.) 

The  sludge  produced  has  generally  been  given  heretofore  in 
cubic  yards.  In  England  it  is  more  customary  to  mention  the 
product  by  weight  and  as  this  is  also  frequently  done  in  the 
United  States  the  following  equivalents  may  be  found  useful, 
although  these  are  subject  to  variation,  depending  on  the  char- 
acter of  the  ingredients  and  the  space  occupied  by  air  after 
draining. 


254 


SEWAGE  SLUDGE 


TABLE  XLI 
APPROXIMATE  WEIGHT  OF  A  CUBIC  YARD  OF  SLUDGE 


Per  cent,  moisture 

Pounds 

Tons 

100 

1685 

0.84 

95 

1695  to  1705 

0.85 

90 

1720  to  1775 

0.86  to  0.88 

85 

1750  to  1820 

0.87  to  0.91 

80 

1790  to  1865 

0.89  to  0.93 

In  selecting  the  method  of  treatment  the  cost  is  an  important, 
and  sometimes  the  controlling,  factor.  The  septic  tanks  at 
Washington,  Pa.,  cost  $4173  per  million  gallons  treated  daily 
or  $15,650  per  million  gallons  gross  capacity,1  while  the  corre- 
sponding costs  for  the  larger  Columbus,  O.,  tanks  (the  contract 
price  for  which  was  particularly  favorable)  were  $3340  and 
$8320,  respectively.  Rectangular  Emscher  tanks  with  3  hours' 
retention  of  sewage  and  5  months'  retention  of  sludge  would 
probably  cost  from  $5000  to  $7000  per  million  gallons  daily 
flow  or  from  $30,000  to  $40,000  per  million  gallons  gross  capacity, 
depending  on  the  excavation.  The  following  figures  on  a  per 
capita  basis  are  given  by  Mr.  George  M.  Wisner,  Chief  Engineer 
of  the  Chicago  Sanitary  District.2 

TABLE  XLII 

COMPARATIVE  COST  OF  SETTLING  TANKS  BASED  ON  A  SEWAGE   FLOW    OF 
200  GALLONS  PER  CAPITA  DAILY 


Type 

City 

Nominal  period 
of  settling 

Cost  per  capita 
for  construction 

Straight-flow 

Columbus  O 

6  hours 

$0.58 

Straight-flow 

Columbus  O 

8  hours 

$0.77 

Dortmund  tank  
Emscher  tank  .  . 

Gloversville  N.  Y  
Atlanta  Ga  .  . 

4  hours3 
3  hours3 

$0.84 
$1.44 

In  case  the  area  available  for  sludge  drying  is  limited  or  costly 
the  Emscher  tank  has  a  decided  advantage,  as  fully  explained 


1  According  to  Mr.  D.  M.  Belcher,  Assoc.  M.  Am.  Soc.  C.  E. 

2  Eng.  Rec.,  Nov  4,  1911. 

3  Sludge  storage  not  considered. 


SUMMARY  AND  CONCLUSION  255 

by  Spillner  and  Blunk.  As  a  result  of  the  Chicago  Experiments 
Mr.  Pearse  is  of  the  opinion  that  with  G  to  8  hours'  retention  of 
sewage  in  a  septic  tank  the  sludge  requires  at  least  20  days  to 
become  spadable,  whereas  with  but  from  1  to  3  hours'  retention 
of  sewage  in  an  Emscher  tank  the  sludge  is  in  condition  to  be 
handled  in  about  5  days,  requiring,  therefore,  not  more  than 
one-fourth  the  area.  For  plain  settled  sludge  a  still  larger  area 
is  required  amounting  to  from  1  to  2  acres  per  1000  tons  if  air 
dried,  or  from  0.2  to  0.4  acres  per  1000  tons  if  buried.  See 
page  239. 

It  is  concluded  that  the  land  required  for  Emscher  tanks 
amounts  to  0.63  sq.  ft.  per  capita  or,  with  appurtenances, 
W  sq.  ft.  per  capita;  and  that  for  the  drying  beds  there  should 
be  provided  0.3  sq.  ft.  per  capita  or,  including  tracks,  dikes  and 
distribution,  0.5  sq.  ft.  per  capita.  The  cost  of  the  beds  is  esti- 
mated at  15  cents  per  capita.1 

Experience  in  the  Emscher  District  has  indicated2  that  three- 
fourths  acre  of  land  is  required  for  every  10,000  persons,  producing 
about  30  cu.  yds.  of  spadable  sludge  (less  than  10  per  cent,  of 
the  volume  of  the  fresh  sludge)  per  annum.  One  man  can 
handle  the  sludge  from  three  times  the  above  population  if  the 
point  of  deposit  is  near  the  plant. 

As  to  the  final  disposition  of  the  sludge  the  method  selected 
depends,  aside  from  the  cost  of  land,  on  the  character  of  the 
sludge,  the  material  available  for  sludge  beds,  the  proximity 
of  dwellings  and  the  general  character  of  the  actual  and  pros- 
pective development  in  the  neighborhood. 

In  general  terms,  perhaps  the  following  selection,  as  proposed 
by  Kinnicutt,  Winslow  and  Pratt,  is  as  appropriate  as  can  be 
given  without  going  into  greater  detail: 

1.  In  the  case  of  small  isolated  plants  air-drying  on  the  land 
or  in  lagoons  is  generally  preferable,  giving  the  dried  sludge  to 
farmers  or  burying  it  in  the  ground. 

2.  For  larger,  but  moderate-sized  plants,  burying  in  trenches 
is  found  satisfactory. 

3.  For  large  cities  located  on  the  coast  the  cheapest  and  most 
expeditious  method  is  removal  by  scow  or  steamer  and  dumping 
at  sea. 

4.  For  large  inland  cities  mechanical  drying  is  often  necessary, 

• "  Rep.  on  Sewage  Disposal."     George  M.  Wisner.  Chicago,  1911. 
2Charles  Saville,  Jour.  Assoc.  Eng.  Soc.,  July,  1911. 


256  SEWAGE  SLUDGE 

in  which  case  the  product  can  be  given  away  as  a  fertilizer  or  it 
can  be  buried  or,  in  isolated  localities,  used  for  filling  in  land. 
If  these  methods  of  disposal  are  not  feasible  for  any  reason  the 
product  can  be  mixed  with  house  refuse  or  with  a  small  amount 
of  coal  and  burned  in  a  destructor. 

At  the  present  time  there  are  over  330  municipal  sewage 
treatment  plants  in  the  United  States.  Of  these,  about  three- 
fifths  employ  the  septic  tank,  either  for  the  complete,  or  as  a 
preliminary  process  and  one-fifth  employ  plain  sedimentation. 
The  former  method,  which  might  more  properly  be  called  the 
semi-septic  process,  has  been  very  generally  adopted  in  the  middle 
west  during  the  past  10  years.  Although  the  term  septic  has 
been  popularly  attached  to  these  tanks  they  are  not  true  septic 
tanks  in  the  light  of  the  Saratoga  decision.  Their  effluents  often 
contain  dissolved  oxygen  and  aerobic  conditions  undoubtedly 
often  exist  in  those  parts  of  the  tank  through  which  the  clearer 
liquid  passes,  while  the  solids,  detained  by  efficient  baffling  and 
generally  collecting  largely  in  the  scum  by  reason  of  the  entrained 
gas,  may  at  the  same  time  develop  septic  or  anaerobic  conditions. 

The  period  of  retention  is  generally  comparatively  brief — often 
not  over  4  hours — so  that  the  sewage  does  not  become  thoroughly 
putrefactive  or  devoid  of  dissolved  oxygen  before  passing  off. 
These  tanks  and  those  devised  by  Travis  and  Imhoff  are  similar 
in  this  respect  and  differ  from  the  septic  tank  of  Cameron,  where 
the  sewage  is  retained  at  least  12,  and  oftener  24  hours.  This, 
too,  is  the  usual  practice  in  operation  in  the  Eastern  states.  The 
divergent  results  obtained  in  the  former  tanks,  for  which  the 
term  "hydrolytic"  has  been  used,  from  those  obtained  with  the 
true  septic  tank  has  resulted  in  a  certain  confusion  of  ideas  in 
regard  to  the  efficacy  and  offensiveness  of  the  septic  tank  pro- 
cess. The  shorter  period  of  retention,  combined  with  a  sewage 
both  fresh  and  weak,  results  in  an  almost  entire  freedom  from 
offensive  odors  in  many  of  the  western  plants  that  is  usually  not 
enjoyed  where  a  strong  sewage  is  retained  for  an  entire  day  in  an 
uncovered  tank.  So,  too,  there  appears  to  be  a  marked  difference 
in  the  amount  of  sludge  and  scum  produced;  for,  as  noted  by 
Mr.  J.  W.  Alvord,  the  deposits,  requiring  removal  from  the 
western  plants  handling  domestic  sewage  only  are  frequently 
very  small  in  amount,  while  the  scum  forms  rapidly,  after  septic 
action  is  established,  to  a  very  considerable  thickness. 

This  suggests  the  desirability  of  studies  to  determine  the  best 


SUMMARY  AND  CONCLUSION  257 

way  of  removing  and  disposing  of  the  scum,  which  differs  materi- 
ally in  character  from  ordinary  sluclge. 

Although  in  the  Emscher  District  the  scum  does  not  seem  to 
accumulate  to  a  great  thickness  it  may  cause  trouble  in  Emscher 
tanks  through  its  buoyancy  by  clogging  or  overlapping  the  vent 
openings  unless  these  are  of, ample  width.  By  breaking  up  the 
scum  occasionally  with  a  rake  much  of  it  will  sink  as  a  deposit 
with  the  sludge  and  release  any  accumulation  of  contained  gas. 

\Yhen  removed  from  the  surface  of  a  tank  receiving  fresh 
sewage  and  whose  contents  are  not  thoroughly  septic  this  scum  is 
not  particularly  offensive  and  may  often  be  dried  out  on  beds  in 
the  open  air  before  final  disposal  if  not  in  the  immediate  vicinity 
of  dwellings. 

The  absence  of  sulphuretted  hydrogen,  and  objectionable 
odors  generally,  in  the  tanks  of  the  Emscher  Association,  has 
been  received  with  a  certain  amount  of  incredulity.  There 
appears,  with  our  present  knowledge,  no  good  reason  why  these 
gases  should  not  form  in  one  style  of  tank  as  well  as  another, 
provided  the  other  conditions  are  similar.  Possibly  the  motion 
of  the  sludge  particles  caused  by  the  eruption  of  gas  bubbles  and 
the  settlement  and  withdrawal  of  sludge  may  influence  the  forma- 
tion of  these  gases,  but  it  would  seem  to  be  largely  accounted 
for  by  the  fact  that  the  greater  part  of  the  organic  matter  from 
which  sulphuretted  hydrogen  is  produced  remains  in  suspension 
or  in  solution  in  the  sedimentation  chamber  and  passes  out  with 
the  effluent,  while  in  the  true  septic  tank  these  are  retained  in  the 
tank  until  putrefaction  is  energetic  and  the  odors,  which  are 
chiefly  derived  from  the  non-sedimentable  portion  rather  than 
from  the  sludge,  are  given  off  in  large  amounts. 

With  regard  to  sludge  disposal  in  America,  while  there  are 
isolated  examples  of  lagooning,  drying  on  the  land,  centrifuging, 
pressing  and  burning,  these  are  so  few  in  number,  or  else  have 
been  carried  on  with  so  little  knowledge  or  care  for  the  highest 
efficiency,  that  no  generalizations  can  be  drawn  that  would 
compare  in  value  with  those  derived  from  foreign  plants  and 
described  so  fully  in  the  reports  of  the  Royal  Commission  on 
Sewage  Disposal  and  by  the  authors  of  the  first  three  parts  of 
this  volume.  The  quite  common  use  of  the  septic  tank  has,  in  a 
measure,  simplified  the  sludge  problem  and  with  the  anticipated 
adoption  of  the  Emscher  tank  by  many  towns  within  a  short  time 
another  step  forward  will  have  been  taken.  Horizontal  tanks, 

17 


258  SEWAGE  SLUDGE 

with  or  without  chemicals,  will  probably  continue  to  be  used  on 
account  of  local  conditions  and  it  is  probable  that  a  broader 
field  for  fine  screening  and  drying  by  centrifugals  will  develop, 
but  from  the  marked  advantages  in  sedimentation  processes 
carried  on  in  conjunction  with  a  special  sludge  chamber  it  seems 
probable  that  the  Emscher  tank  in  its  present  or  a  modified  form 
is  destined  to  play  an  important  part  in  sewage  treatment  in 
America  for  some  time  to  come. 


LOCALITIES 

PAOB 

Accrington 19,  63 

Allenstein ' 78,136 

Alliance 233 

Andover 211,  217,  248 

Aschersleben 132 

Atlanta 209 

Aylesbury 244 

Baltimore 209,  237 

Beckum 146 

Belfast 128 

Berlin 10,  100,  127,  133 

Biebrich 60 

Bielefeld 136 

Birmingham 11,  12,  19,  87,  88,  89,  90,  133,  135,  141,  142,  244 

Blackburn 68 

Bochum    .    .    :    .     145,  164,  173,  177,  179,  181,  182,  183,  185,  189,  226,  229 

Bockenheim 93 

Bolton 34,  47 

Boston 196,  197,  199,  200,  202,  203,  204,  239 

Bradford 100,  108,  245 

Braunschweig 132,  133 

Bremen 18,  33,  57 

Brieg 18,  63 

Brockton       .    .    .    : 209,  242 

Brooklyn '. 235 

Briinn .    131 

Bury      20,  47,  68,  100,  128 

California      242 

Canton .233 

Cassel     ....     10,  18,  31,  59,  63,  92,  106,  107,  124,  125,  133,  136,  137,  183 

Charlottenburg 10,  17,  43,  45,  48,  100,  124,  135 

Chemnitz 70 

Chicago 196,  197,  225,  226,  241,  254,  255 

Chorley 20,  68 

Colchester 68 

Cotne 19 

Cologne 17,30,38,40,92,126,131 

Columbus      .    .   78,  100,  110,  196,  197,  201,  204,  212,  214,  217,  219,  239,  254 

Coney  Island 235 

Copenhagen      128 

Copenick 11,21,28,62,99,127,133 

Coventry 244 

259 


260  LOCALITIES 

PAGE 

Culmsee 18,  19 

Deutz 104 

Dorchester 202,  203,  204 

Dresden 17,  104 

Dublin 110,  238,  245 

Dusseldorf 26 

Ealing 68 

East  New  York 235,  236 

Edinburgh 123 

Elberfeld  ...      10,  17,  26,  28,  38,  40,  41,  52,  57,  58,  63,  97,  109,  133,  134 

Elbing ; 106 

Elmhurst 241 

England     .    .    3,  16,  37,  64,  68,  69,  88,  89,  92,  94,  100,  121,  133,  134,  137, 

243,  253 

Essen 20,  133,  134,  144,  145,  147,  161,  163,  164,  165,  167,  168, 

169,  170,  173,  175,  176,  177,  180,  181,  182,  183,  184, 
185,  186,  188,  189,  226,  227 

Failsworth 18 

Frankfort  .    .    .    .  5,  10,  17,  18,  26,  31,  57,  60,  63,  68,  70,  71,  73,  79,  85,  86, 

88,  91,  93,  97,  100,  102,  107,  124,  125,  126,  128, 

131,  132,  133,  134,  137,  138,  139,  161, 

183,  245 

Gatow .124 

Germany 5,  16,  61,  65,  106,  121,  135,  137,  203 

Glasgow 20,  94,  110,  238 

Gottingen 136 

Guildf  ord 20 

Guben 52 

Halberstadt 19,  20,  57,  62,  63 

Halifax 138 

Halle ' 66 

Hamburg 17 

Hampton 19,  135,  142,  147 

Hanover 7,  17,  18,  31,  51,  71,  73,  74,  76,  98,  129,  138,  246 

Harburg 29,  71,  73,  74,  125,  126,  138,  161,  183 

Hendon 20 

Heywood 47 

Holzwickede 177 

Huddersfield 99,  133 

Hyde 128 

Insterburg 136 

Karlsruhe 28 

Kingston 94,  123 

Konigsberg 133 

Langensalza 18,  63,  132 

Lawrence       196,  197,  210,  217 

Leeds 12,  23,  63 

Leipzig  '.' 6,  17,  18,  19,  20,  23,  56,  57,  60,  91,  109,  124,  137 


LOCALITIES  261 

PAGE 

Litchfield 20 

London 6,  15,  20,  110,  128,  129,  238 

Luttich 125,  183 

Madison 219 

Mairich 45,  52,  132,  134 

Manchester 12,19,88,89,110,126,127,128,129,142,238 

Mannheim 10,  18,  31,  51,  57,  70,  85,  86,  88,  89,  91,  135 

Mansfield 221,240 

Marburg 17,  26 

Massachusetts 197,  242,  246 

Merseburg 19,  20,  26,  52 

Munich-Gladbach 17,  18,  28,  31,  51,  63 

Mulheim-Ruhr 142 

Mullheim 19,  63 

Neustadt 45,  50,  91 

Neustrelitz 62 

New  Brunswick 209 

New  York 241,  246 

Norwich 142 

Oberschonenweide 67,  104,  105 

Ohrdruf 17,  52,  63 

Oldham 126 

Oppeln 52 

Osdorf 10 

Paterson 197 

Paris 7 

Pawtucket 210,  221 

Pforzheim 128 

Philadelphia.    .    .    .196,  197,  206,  214,  224,  226,  240,  246,  247,  250,  253 

Plainfield 197,  210,  217,  221 

Potsdam 94,  98,  99,  127,  137,  138 

Providence 197,  209,  232,  239,  243 

Queens 241 

Rauxel 146 

Reading 206,  207,  217,  240,  245 

Recklinghausen    .    .     59,  92,  135,  144,  154,  155,  157,  158,  161,  164,  170, 

173,  175,  177,  179,  180,  183,  185,  189,  226 

Remscheid 132,  133,  134 

Rheydt      24 

Rochdale 63 

Salford 11,  128,  129,  238 

Saratoga 223,  256 

Schoneberg 17 

Sheepshead  Bay 235 

Sheffield 12,  20,  21 

Siegen 24,  52 

Skegness 78 

Southampton   ....  110,  238 


262  LOCALITIES 

PAGE 

Spandau 66,  70,  99,  137 

Stargard 18 

Stuttgart 19,  63,  98,  102 

Tegel 102,  137 

Thorn 39 

Torgau 91 

Unna 12,  19,  24,  63,  92,  142 

Washington       254 

Waterbury    . 196,  197,  202,  209,  222,  239 

White  Plains ! 234,  24f) 

Willesden      68 

Wimbledon 68,  133,  134 

Windsor 244 

Worcester 196,197,200,212,230,243,247,249 


LIST  OF  NAMES 

PACK 

Alvord . 256 

Ashton       129 

Barwise 12 

Bechold 125,  121 

Beck 106 

Belcher 254 

Bemmelen,  van 133 

Blunk 164,  165,  172,  228,  255 

Bosley % 237 

Bowles .    .    .   219 

Bredtschneider 97,  127,    131 

Brown 204 

Bujard 127 

Busing • 16 

Butschli,  von 133 

Cameron 256 

Carpenter      210 

Clapp 232 

Crimp 244 

Davis 219 

Degener 106,  127 

Dost 98,  104,  127 

Dunbar 27,  68,  131,  133,  135,  140 

Egestorff 71 

Eisner 227,  252 

Favre .   183 

Fellner 93 

Fidler 47 

Frank 127 

Friedrich 126 

Fuller 196,  197,  198,  212,  243 

Gault 201 

Gavet 197 

Geiger 28 

Gohring '.127 

Gregory 227,  229 

Grimm 39,  49 

Grosse  Bohle 126,  131 

Grossmann 126 

Haack  .    .  • 125 

Haubold 70 

Heine 126,  127 

263 


264  LIST  OF  NAMES 

PAGE 

Henkel 106 

Hering 227 

Herzberger 128 

Honig 137 

Hopfner 124,  125 

Horsfal      128 

Imhoff        64,  131,  132,  147,  172,  224,  229,  256 

Jackson - 220,  236 

Johnson 197,  212 

Kinnicutt,  Winslow  and  Pratt 197,  200,  217,  239,  246,  255 

Kolle 128 

Koschmieder 96,  104,  127 

Krupp 166 

Kubel 156 

Kuichling      ?    .    .    .    .   202 

Lacombe 125 

Lathbury-d'Olier 245 

Liibbert 140 

Metzger 117,  121 

Middeldorf 147 

Paulmann 124,  125 

Pearse 225,  255 

Phelps 199,  200 

Poschl 133 

Powers 234,  235 

Proskaner 97 

Reischle 69,  73,  77,  98,  104,  125,  127,  138,  161 

Reuther      51 

Rideal 244 

Rothe     . 98 

Rothe-Degener 127 

Rothe-Rockner 134 

Ruggles 239,  243,  244,  246 

Saloman 40,  124,  132,  133,  136 

Saville      227,  241 

Schaefer-ter  Meer 71,  244 

Schiele 69,  94,  128,  131 

Schmeitzner 27,  70 

Schonfelder         38,  40 

Schreiber 10 

Schury 127 

Schwerin,  von 79,  139,  177 

Scotland      123 

Snow        242 

Spillner 79,  165,  172,  178,  179,  183,  185,  186,  227,  228,  255 

Steuernagel 14 

Taylor 209 

Thiesing      69,  73,  77,  125,  138,  161 


LIST  OF  NAMES  265 

PAGE 

Thumm 131 

Tillmans      80,  131,  139 

Travis          49,  135,  142,  147,  256 

Uhlf  elder 128 

Ulrich      208 

Voelcker 242 

Voss 125,  126 

Wattenberg 147 

Weand 206,  207,  208 

Webster 197 

Wegner 52 

Whipple 197,  198 

Winslow      200 

Winter 164 

Wisner 225,  246,  254 

Ziegler 93 


INDEX 


ABC    Process  (See  also   Chemical 

Precipitation),  94,  123,  244 
Acreage  required    (See  Area,  Sludge 

Beds,  Sludge    Drying, 

Sludge  Burial.) 
Aerobic    (See  Bacteria.) 
Agricultural  use     (See  Fertilizer.) 
Air     (See  Compressed  Air.) 
Albuminoid  Ammonia,  211,  230,  232 
Alum,    Alumina     (See    Sulphate    of 

Alumina.) 
Alumino  ferric,  68 
Ammonia,  140 
Anaerobic     (See  Bacteria.) 
Analysis     (See  Sludge  Composition, 

Sewage.) 

Angle     (See  Slope.) 
Area     (See  also  Sludge  Beds.) 

for  sludge  beds,  109,  166,  240, 

254,  255 
for  sludge  disposal,  88-,  89 

Bacteria,  24,  79,  160,  164 
aerobic,  140 
anaerobic,  140 
pathogenic,  24 

Bacteria  beds     (See  Contact  Beds.) 
Baffle,  215,  224,  225,  256 
Belloform,  132 
Benzine,  107,  125 
Boston  Main  Drainage,  200,  239 
Briquettes,  80,  93,  97,  99,  100,  102, 

127 

Broad  irrigation     (See  Irrigation.) 
Bubbles     (See  Gas.) 
Burial     (See  Sludge  Burial.) 
Burning,   disposal    by,    94,  95,  99; 
sludge,     100,    114,   128,   241, 
242,  244,  245,  249,  250,  251, 
257     (See    also    Sludge, 
Calorific  Value.) 


Calorific  value  (See  Sludge,  Cal- 
orific Value.) 

Candy  system,  47 

Carbon  dioxide  (See  Carbonic 
Acid.) 

Carbonic  acid,  97, 101,  143,  191,  247, 
248 

Carriage,  Carting  (See  Conveyor, 
Channels,  Pipes,  Transpor- 
tation, Wagon.) 

Cellulose,  125 

Cement,  99 

Centrifugal  machine,  69,  112,  113, 
129,  137,  138,  142,  208,  244 
258 

Centrifuged  sludge,  100,  106,  138, 
161,  208  257 

Cesspool,  52,  85,  190,  237 

Channels,  50,  53,  61,  85,  86 

Chemical  precipitation,    10,   20,   21, 

115,  133,  135,  230,  243,  244 

sludge,   67,  84,    104,  230,   244, 

252,  253 
tank,  232,  233,  235 

Chicago     Sanitary     District,      225, 
254 

Chlorine,  235 

Cinders     (See  Slag.) 

Clay,  109 

Cleaning  tanks,  13,  15,  31,  33,  34,  48, 
217,  233  (See  also  Re- 
moval.) 

Cleaning  grit  chambers,  22,  52 

Cleaning  screens,  204,  207 

Clinker     (See  Slag.) 

Coal,  97,  98,  100,  104,  114,  127,  190, 
235,  241,  248,  249,  250,  251 

Coke,  165,  248,  249 

Colloids,  39,  79,  132,  141 

Combustion  (See  Burning,  Gas, 
Sludge-Calorific  Value.) 


267 


268 


INDEX 


Composition  (See  Detritus,  Gas, 
Screenings,  Scum,  Sewage, 
Sludge.) 

Compost,  92,  136 
Compressed  air,  49,  51,  52,  66,  87 
Conduits     (See  Channels,  Pipes.) 
Contact  beds     (See  Filtration.) 
Conveyor,  52,  76,  80,  207,  245 
Copperas,  230 

Cost     (See  Process  in  Question.) 
Crops,  237,  238,  242 

Decomposition     (See  Digestion.) 
Delivery     (See  Conveyor,  Channels, 

Pipes,  Transportation, 

Wagon.) 
Deodorant,  60 
Detritus     (See  also  Screenings.) 

from  grit  chambers,  8,  22,  83, 

92,  115,  199 

amount,  17,  200,  201,  202 
Digestion  of  sludge,  11,  14,  78,  112, 

140,  141,  143,  160,  177,  178, 

182,  191,  218,  219,  226,  227, 

228,  247 

Dilution  of  sludge,  74 
Disinfectant,  7 
Disinfection,  235,  236 
Distillation  of  gas,  101,  248,  249 
of  grease,  104,  107,  108,  248 
Distribution  (See   Sludge-Transpor- 
tation.) 
Dortmund   tank,    37,    39,    48,   252, 

254 
Drainage  water,  151,  152,  156,  157, 

159,  160,  169,  178,  188,  191, 

250 

Dredge,  26,  34,  50 
Drying  beds     (See  Sludge  Beds.) 
Dumping  at  sea.     (See  Ocean  Dis- 


on  land     (See  Filling  in  Land.) 

Ejector     (See  Steam  Ejector,  Shone 

Ejector.) 

Electro-osmose,  79,  139 
Emscher  tank,  14,  19,  39,  42,  45,  53, 

59,63,79,99,112,116,143, 


147, 177,  178,  180,  183,  191, 
224,  243,  255,  257,  258 
sludge,  62,  143,  159,  160,  179, 

224,  241,  243,  252,  253 
cost  of,  189,  254 

Enzymes,  160,  164 

Essen  tank     (See  Emscher  Tank.) 

Facilol,  60,  132 

Fats     (See  Grease.) 

Fermentation     (See  Digestion.) 

Ferric  sulphate,  230 

Fertilizer,  3,  5,  6,  55,  80,  83,  84,  85, 

89,  91,  92,   107,   108,    114, 

123,  125,  126,  170,  177,  183, 

184,  233,  235,  236,  242,  246, 

252 

Fidler  system,  48 
Filling  in  land,  83,  109,  170,  231,  240, 

252,  256 

Filter  pressing  (See  Sludge  Pressing.) 
Filters,  intermittent,    78,    113,    140, 

230,  242 
contact,  4,  12,  19,  27,  57,  74,  78, 

112,  113,  140,  252 
sprinkling,  4,  19,  57,  141,  208, 

252 

drum,  137 
roughing,  141 
Flies,  59,  60,  85,  92 
Floating  arm,  28,  230 
Flow,  31,  32,  42,  45,  53,  178,  190,  225 
Flushing,  32,  42,  45,  53 
Fresh    sludge    (See    Sedimentation, 

Sludge.) 

Gas  (See  also  Odors),  12,  25,  39, 
57,  78,  79,  95,  96,  97,  101, 
127,  141,  146,  153,  176,  177, 
179,  182,  183,  184,  187,  191, 
202,  217,  224,  225,  227,  247, 
256,  257 
composition,  102,  105,  143,  161, 

225,  248,  249 

calorific   value,    102,    103,    105, 

114 

Gels     (See  Hydrogels.) 
Globe  fertilizer,  94 
Gradient     (See  Slope.) 


INDEX 


269 


Grease,  230 

in  scum,  216 

in  sewage,  4,  25,  84,  106,  113 
in  sludge,  10,  24,  36,  41,  67,  70, 
73,91,95,97,100,106,113, 
114,  125,  183,  248 
value,  108,  126 

Grit,    Grit   chamber  (See   also   De- 
tritus), 8,  19,  22,  52,  78, 165, 
180,  200,  201,  202,  204,  225, 
252,  253 
Ground  water,  230,  233 

Heat  (See  Burning,  Calorific  Value.) 
Hille's  process,  244 
Hydrogels,  133,  160 
Hydrogen     sulphide     (See    Sulphu- 
retted Hydrogen,  Odor.) 
Hydrosols,  132 

Illuminating  power,  102 
Imhoff  tank     (See  Emscher  Tank.) 
Incineration     (See  Burning.) 
Infiltration  to    sewer     (See  Ground 

Water.) 

Intermittent  filtration     (See  Filtra- 
tion.) 

Iron  salts  as  precipitants,  68,    137 
Irrigation,    12,  74,   83,  88,  90,    112, 
113,  114,  132,  133,  140 

Kremer   apparatus,   10,   24,  36,  43, 
45,  48,  61,  84,  108,  109,  116 

Lagoons     (See  Sludge  Beds.) 
Land  (See  Acreage,  Filtrati  on ,  Irriga- 
tion, Filling  in  Land.) 
Lignite  process,  11,  21,  44,  45,  47,  62, 

67,  69,  70,  98,  100,  104,  115, 

127,  133,  137 
sludge,  94,  99,  105,  138 
Lime  precipitant. 

added  to  sewage,  10,  20,  84,  106, 

109,  135,  230,  232,  233,  235, 

236,  244 
added  to  sludge,  3,  59,  60,  68, 

69,  70,  84,  92,  94,  99,  137, 

230,  231,  233,  242 
Loam,  109 


Manganese  dioxide,  235 
Marsh  gas     (See  Methane.) 
Massachusetts  Institute  of  Technol- 
ogy Experiments,  199,  202 
Massachusetts      State      Board      of 
Health,  210,  217,  230,  247 
Methane,  140,  143,  179,  191,  247,  248 
Metropolitan  Sewerage  Commission, 

Boston,  204,  205 
of  New  York,  214,  229,  235 
Micells,  133 

Moisture    in    sludge     (See    Sludge- 
composition.) 
Montejus,  106 

Native  Guano,    94,    123    (See    also 

Fertilizer.) 
Night   soil,   5,    85,    236     (See    also 

Fertilizer.) 

Nitrates,  140,  152,  156,  188 
Nitrites,  140,  152,  156,  188 
Nitrogen,  6,  83,  84,  85,  93,  114,  169, 

183,  242,  247,  248,  249 
Nuisance      (See   also   Odor,   Flies), 

3,  121,  125,  256 

Ocean  disposal  of  sludge,  5,  6,  110, 

128,  238,  255 
Odor,  5,  11,  31,  36,  52,  56,  59,  60, 

61,  67,  69,  77,  79,  80,  85,  88, 

92,  93,   94,   101,   109,   129, 
132,  134,  135,  136,  141,  152, 
159,  179,  183,  190,  191,  217, 
222,  225,  227,  237,  240,  252, 
256,  257 

Odorless  excavator,  237 
Oxygen,  101,  256 

Pathogenic    germs     (See  Bacteria.) 
Peat,  59,  60,  92,  98,  102,  114,  127, 

132,  248,  249, 

Perchloride  of  iron,  235,  236 
Phosphates,    Phosphoric    acid,    83, 

93,  242,     248     (See    also 
Fertilizer.) 

Pipe  (See  also  Transportation,  Flow), 
42,  47,  49,  52,  53,  61,  85, 
86,  227,  252 

Plowing  under     (See  Sludge  Burial.) 


270 


INDEX 


Pneumatic  (See  Compressed  Air, 
Odorless  Excavator,  Sludge 
Transportation,  Vacuum 
Receiver.) 

Population     (See  City  in  Question.) 

Potash,  83,  84 

Poudrette,  93,  125 

Power  required,  76 

Precipitant  (See  Alumino-f  erri  c, 
Chemical  Preci  p  i  t  a  t  i  o  n, 
Ferric  Sulphate,  Iron  Salts, 
Lignite,  Lime,  Sulphate  of 
Alumina.) 

Pressed  sludge  (See  Sludge  Cake.) 

Pressing  sludge  (See  Sludge  Pressing.) 

Pump,  pumping  sewage,  225 

sludge,  41,   50,'  51,  66,  87,  135, 

146,  230,  233,  235,  237 
detritus,  26,  27,  210 

Putrefaction,  7,  14,  25,  59,  76,  135, 
143,  169,  180,  236,  256,  257 

Recovery      (See      Fertilizer,     Gas, 

Grease,   Calorific  Value.) 
Removal     of     detritus     from     grit 

chambers,  22,  25,  201 
from  screens,  23 
of  sludge,  22,  129,  242 
from  plain  precipitation,  23,  27, 

47,  48,  215,  217 
from      chemical     precipitation, 

235 
from    Emscher   tanks,  23,  143, 

170,  174,  227 
from  septic  tanks,  23,  218,  221, 

222,  223 

Retention     (See  Removal.) 
Revenue     (See   Sludge,    Value  of.) 
Roily  water     (See  Turbid  Liquid.) 
Rotary      dryer      (See      Centrifugal 

Machine.) 

screen     (See  Screen.) 
Rothe-Rockner  process,  134 
Royal   Commission  of  Sewage  Dis- 
posal, 238,  242,  244,  257 
Experiment    Station,   Berlin, 
16,  128 

Salt,  235 


Sand  (see  also  Filtration,  Detritus), 
24,  26,  109,  241 

Sand  filters  (See  Filtration,  Inter- 
mittent.) 

Sawdust,  241 

Scraper,  45,  47,  73 

Screw  conveyor     (See   Conveyor.) 

Screen,  screening:  bar,    19,  22,   51, 

84,  115,  202,  203,  204,  209, 

210,  221,  225,  233,  235,  252 

mesh,  22,  76,  84,  115,  200,  204, 

206,  208,  209,  245,  252,  258 

Screenings,  9,  92,  202 

composition,   9,   204,   205,   209, 

210,  245 

amount,  17,  19,  204,  205,  209, 
210,  245,  253 

Scum,  110,    141 

composition,  216,  224,  225 
amount,  215,  216,  222,  224,  227, 

256,  257 
disposal,  126 

Scum  board,  29,  141,  215,  252 

Sea  discharge     (See  Ocean.) 

Sedimentable  matter,  190 

Sedimentation     (See  also  Precipita- 
tion.) 
plain,  15,  18,  30,  190,  210,  212, 

213,  256,  258 

tank,  40,  46,  62,  115,  133,  140, 

141,  142,  212,  214,  217,  257 

sludge,  62,  64,  78,  83,  102,  131, 

132,  214,  217,  241,  252,  253 

Septic  tank  (See  also  Sedimenta- 
tion, Sludge),  11,  25,  74, 
79,  84,  115,  140,  141,  142, 
217,  219,  221,  222,  223,  247, 
252,  253,  254,  256 

Settling     (See  Sedimentation.) 

Sewage,  composition,  8,  11,  16,  20, 
195,  196,  197,  198,  199,  206, 
211,212,213,218,220,  221, 
222,  223,  226,  231,  234,  236, 
247 

Shone  ejector,  230 

Shutters,  50 

Siphon,  47,  53,  154 

Skimmer,  44,  45 

Slag,  99,  101,  109,  152,  164 


INDEX 


271 


Slope  of  bottom,  30>  31,  37,  38,  39, 
40,  41,  43,  49,  50,  224 

of  pipe,  49,  53 

Sludge  bed  (see  also  Sludge  Dry- 
ing, Area),  56,  57,  59,  63, 
87,  90,  112,  134,  136,  137, 
147,  152,  159,  164,  165,  175, 
177,  188,  191,  235,  237,  240, 
241,  243,  250,  254,  255,  257 

burial,  88,  135,  142,  233,  239, 
243,  255  (See  also  Area, 
Sludge  Drying.) 

cake,  67,  94,  99,  100,  106,  107, 
108,  231,  233,  244,  252 

calorific  value,  95,  96,  98,  100, 
102,  104,  127,  152,  246,  249, 
250,  251,  252  (See  a  Iso 
Burning.) 

composition,  7,  9,  10,  11,  12,  13, 
14,  16,  36,  43,  54,  61,  74,  84, 
89,  91,  122,  131,  144,  145, 
146,  148,  149,  151,  153,  154, 
158,  159,  161,  167,  168,  175, 
180,  181,  184,  185,  212,  214, 
215,  217,  218,  221,  222,  223, 
224,  226,  242,  247,  250 

cylinder,  43 

distribution  (See  Pipe  Lines, 
Channels.) 

drainage  of,  3,  41,  56,  91,  134, 

137,  142,  146,  160,  163,  164, 
176,  178,  186,  188,  191,  241 
(See   also    Sludge    Drying, 
Sludge  Beds.) 

drying,  55,  128,  129,  132 

artificial,    64,    94,    104,    122, 

138,  244,  252,  255,  257,  258 
in  the  air,  6,  21,  24,  30,  36,  56, 

60,  61,  62,  67,  80,  90,  93, 
97,  98,  107,  109,  122,  133, 
140,  156,  176,  212,  224,  227, 
228,  235,  240,  241,  244,  245, 
252,  254,  255,  257 

freezing,  176 

grease  in     (See  Grease.) 

holder  (See  Sludge  Receiver, 
Sludge  Tank,  Vacuum  Re- 
ceiver.) 

liquefaction   (See  Digestion.) 


liquor     (See  Turbid  Liquid.) 

removal     (See  Removal.) 

measuring,  173,  175 

press,  18,  65,  77,  106,  108,  113, 
127,  137,  142,  230,  232,  233, 
243,  244,  252,  257 

pushing  car,  33,  34 

receiver,  66,  72,  78,  86,  87,  110 

sampling,  184 

specific  gravity  (See  Sludge 
Composition,  Weight.) 

septic  tank,  61,  62,  64,  91,  92, 
98,  102,  103,  110,  140,  141, 
142,  247,  248 

steamer  (See  Sludge-Transpor- 
tation, Ocean  disposal.) 

storage  of,  85, 113,  140  (See  also 
Digestion,  Removal.) 

tank,  207,  230 

transportation,  6,  51,  59,  76, 
85,  91,  110,  124,  129,  231, 
232,  237,  238,  239,  241 

value  of,  82,  84,  89,  91,  92,  99, 

123,  124,  126,  129,  170,  242, 
243,  244,  245,  246 

volume,  5,  7,  10,  12,  13,  14,  15, 

19,  54,  61,  64,  70,  73,  74, 
77,   82,   90,    102,    104,    112 

124,  126,  166,  253 

from  plain  sedimentation  102, 
132,  211,  212,  213,  214,  234 

from  septic  tanks,  19,  219, 
220,  222,  252,  256 

from  Emscher  tanks,  144, 
148,  150,  161,  162,  163,  165, 
167,  168,  173,  176,  177,  221, 
224,  225,  226,  227,  228,  229, 
252 

from  chemical  precipitation, 

20,  230,  232,  233,  234,  235, 
236,  240 

weight,  76,  102,  148,  150,  151, 
155,  158,  166,  167,  168,  223, 
231,  244,  250,  253 
Smell     (See  Odor.) 
Solids     in     sewage     (See     Sewage- 
composition.) 

in  sludge  (See  Sludge-compo- 
sition.) 


272 


INDEX 


Sprinkling  filter     (See  Filtration.) 

Squeegee,  34,  35,  47 

Steam  ejector,  26,  52 

Stirring  device,  44,  45,  47,  66 

Storage     (See  Removal.) 

Storm  water,  8,  20,  191 

Street  wash     (See  Storm  Water.) 

Sulphate  of  alumina,  69,  98,  104, 127, 
133,  230,  244 

Sulphide    of    hydrogen.     (See    Sul- 
phuretted hydrogen.)    (See 
also  Nuisance,  Odors.) 
of  iron,  94,  134,  141,  221 

Sulphuric  acid,  106,  108,  235 

Sulphuretted  hydrogen,  79,  140,  143, 
156,  169,  179,  257  (See  also 
Odor.) 

Sump,  29,  42,  50,  51,  53,  78 

Suspended  matter  (See  Sewage- 
composition,  Sludge-com- 
position.) 

Sweepings,  92,  93,  100,  114,  128, 
136 

Tank  (See  Candy  T.,  Chemical 
Precipitation  T.,  Dortmund 
T.,  Emscher  T.,  Kremer 
Apparatus,  Septic  T.,  Sedi- 
mentation T.,  Sludge' T.) 

Temperature,  227,  247,  250 
Towers,  14,  27,  45,  127,  134 
Trade  waste,  206,  232,  252 


Transportation  (See  Carts,  Con- 
veyors, Channels,  Pipes, 
Sludge  Transportation.) 

Turbid  liquid,  27,  29,  36,  38,  48,  49, 

50,  51,  57,  78,  85,  139,  186, 
187,  188,  230,  241 

Under-drainage  (See  Drainage 
Water,  Sludge  Drainage.) 

Utilization  of  sludge,  agricultural 
(see Fertilizer),  burning  (see 
Calorific  Value)  (Burning), 
production  of  gas  (see  Gas), 
recovery  of  grease  (see 
Grease.) 

Vacuum  receiver,  18,  27,  29,  41,  42, 

51,  52,  78 

Valves,  23,  28,  29,  48,  49,  51,  53,  72, 

86 
Velocity,  14,  30,  200,  201,  202,  212, 

215,  233 
Vermin,  125 
Viscosity,  160 

Vitriol     (See  Sulphuric  Acid.) 
Volume  of  sludge     (See  Sludge.) 

of  scum     (See  Scum.) 

Wagon,  52,  85,  129,  237 
Wastes     (See  Trade  Waste.) 
Water  in  sludge     (See  Sludge  Com- 
position.) 

Wells  (see  also  Sump),  14,  27,  29, 
36,  37,  39,  45,  47 


ERRATA 

Page    16.     Line  2  from  bottom.     In    place   of   "suspened"    should    read 

"suspended." 

"       19.     Line  4.     In  place  of  "  100  "  should  read  "  1000." 
"      23.     Line  15.     In  place  of  "  very  "  should  read  "  every." 
"      27.     Reverse  the  numbers  and  positions  of  footnotes. 
"      55.     Under  Fig.   20.     In    place   of    "the   watering"    should    read 

"De-  watering." 

"       "       Last  word.     In  place  of  "incineration"  should  read  "evapora- 
tion." 

"      56.     Line  12.     In  place  of  "presser"  should  read  "presses." 
"      62.     Line  4.     In  place  of  "2  to  3  to  1  to  2"  should  read  "2/3  to  1/2." 
•'•      74.     Line  8.     In  place  of  "  2  to  3  "  should  read  "  2/3." 
Line  9.     In  place  of  "  1  to  3  "  should  read  "  1/3." 
"       "       Line  12  from  bottom.     In  place  of  "9  to  10"  and  "1  to  10" 

should  read  "9/10"  and  "1/10." 
Line  15.     In  place  of  "  1720"  should  read  "  1718." 
"     109.     Line  10.     In  place  of  "withe"  should  read  "with." 
"     135.     Line  5  from  bottom.     In  place  of   "W.  Oven — Travis"  should 

read  "W.  Owen  Travis." 

"  148.  Line  2  from  bottom.  In  place  of  "1.9  "  should  read  "91." 
"  "  Line  1  from  bottom.  In  place  of  "  2.1  "  should  read  "21." 
"  183.  Line  15  from  bottom.  In  place  of  "  Rechlinghausen "  should 

read  "  Recklinghausen." 

"    200.     Line    7.     in  place  of  "were"  should  read  "was." 
"     201.     Line  2.     In  place  of  "  18,150"  should  read  "  1815." 
"     204.     Line   1   from  bottom.     In    place   of    "Com'rs"    should    read 

"Com'n." 

"     210.     Line  13.     In  place  of  "pumpted  "  should  read  "pumped." 
"    219.     Place  Table  XVI  at  foot  of  page. 
"    225.     Line  8.     In  place  of  " 40  "  should  read  " 400." 
"     239.     In  Table  XXIX  indent  "  Barking  "  instead  of  "  Glasgow." 
"     244.     Line  10  from  bottom.     In  place  of  "the  Hilles"  should  read 

"  Hille's." 

"  12  4 
251.     Lines  3  and  4  from  bottom  in   column  2.     In  place  of        ' 

should  read  "  12.4-22.9." 
"     255.     Line  13.     In  place  of  "  10  "  should  read  "  1.0." 


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